DOCSP-36167: Update .NET compatibility with note on source generator model requirements (#3288)

## Pull Request Info

Jira ticket: https://jira.mongodb.org/browse/DOCSP-36167

*Staged Page*

- [Platform and Framework Compatibility - .NET
SDK](https://preview-mongodbcbullinger.gatsbyjs.io/realm/docsp-36167-compatibility/sdk/dotnet/compatibility/)

## Release Notes

- **.NET SDK** 
- Compatibility: Added note with minimum framework requirements when
using the new source model generator.

Author: cbullinger

source/includes/note-java-and-realmany.rst

@@ -1,5 +1,4 @@
-.. note:: New apps cannot use ``RealmAny``
+.. note:: New Java SDK apps cannot use ``RealmAny``
 
     New :atlas:`App Services Apps </app-services/apps/>`
-    will not be able to synchronize data models with properties of type 
-   ``RealmAny``.
+    will not be able to synchronize data models with properties of type ``RealmAny``.

source/sdk/dotnet/compatibility.txt

@@ -11,11 +11,18 @@ Platform and Framework Compatibility - .NET SDK
    :depth: 2
    :class: singlecol
 
-Compatibility
--------------
+.. meta::
+   :description: Compatibility information when developing with Atlas Device SDK for .NET.
+
+.. facet::
+  :name: genre
+  :values: reference
+
+Target Platforms
+----------------
 
 The following table shows which .NET
-version(s) you can use with Realm on your target platforms:
+versions you can use with the SDK on your target platforms:
 
 .. list-table::
    :header-rows: 1
@@ -32,80 +39,108 @@ version(s) you can use with Realm on your target platforms:
    * - Windows 8.1 or later
      - | .NET Core 2.0 or later (.NET 5.0 Framework or later)
        | .NET Framework 4.6.1 or later
-       | MAUI requires Windows 11 and Windows 10 version 1809 or higher, using 
+       | MAUI requires Windows 11 and Windows 10 version 1809 or higher, using
          Windows UI Library (WinUI) 3.
 
    * - Universal Windows Platform (UWP)
      - .NET Standard 2.0 or later (Fall Creators Update)
 
    * - macOS
      - | .NET Core 2.0 or later (.NET 5.0 Framework or later)
-       | Xamarin.Mac for macOS 10.11 or later. 
+       | Xamarin.Mac for macOS 10.11 or later.
        | MAUI requires macOS 10.15 or later, using Mac Catalyst.
 
    * - iOS
-     - | Xamarin.iOS for iOS 9 or later. 
+     - | Xamarin.iOS for iOS 9 or later.
        | MAUI requires iOS 11 or later.
 
    * - Android
-     - | Xamarin.Android for Android 4.1 (API level 16) or later. 
+     - | Xamarin.Android for Android 4.1 (API level 16) or later.
        | MAUI requires Android 5.0 (API level 21) or later.
-  
+
    * - tvOS
-     - | Xamarin and Unity for tvOS 9.0 or later. 
+     - | Xamarin and Unity for tvOS 9.0 or later.
        | For more information on developing for tvOS, see :ref:`<ios-tvos>`.
 
+.. note:: Compatibility with SDK Object Models
+
+  The source generator models in .NET SDK v10.18.0 and later
+  require the following:
+
+  - .NET Core 2.0 or later (.NET Framework 5.0 or later)
+  - C# 9.0 or later
+
+  If you are targeting an older version of .NET Framework, your object models
+  must derive from the
+  :dotnet-sdk:`RealmObject <reference/Realms.RealmObject.html>`,
+  :dotnet-sdk:`EmbeddedObject <reference/Realms.EmbeddedObject.html>`, or
+  :dotnet-sdk:`AsymmetricObject <reference/Realms.AsymmetricObject.html>`
+  base classes required by the old source generator.
+
+  The following demonstrates how you can adjust your current object
+  models for compatibility with older .NET Frameworks:
+
+  .. code-block:: csharp
+
+     public partial class Person : IRealmObject // Current model
+
+     public class Person : RealmObject // Adjusted to inherit from RealmObject
+
+  For more information, refer to :ref:`dotnet-define-a-realm-object-schema`.
+
 Development Environments
 ------------------------
 
-You can use the following development environments to build Realm apps with 
+You can use the following development environments to build apps with
 the .NET SDK:
 
 - Visual Studio 2015 Update 2 or higher for Windows
 - Visual Studio for Mac 7.0 or higher
-- Unity `2020.3.12f1 (LTS) <https://unity3d.com/get-unity/download/archive>`__ 
+- Unity `2020.3.12f1 (LTS) <https://unity3d.com/get-unity/download/archive>`__
 
 .. note::
 
-    The :doc:`Realm .NET SDK </sdk/dotnet>` may be compatible with
-    other versions of Unity, but ``2020.3.12f1 (LTS)`` is the version that the
-    Realm team uses for testing and development. We recommend using
-    this version to ensure your project works with Realm and that
+    The :doc:`.NET SDK </sdk/dotnet>` may be compatible with
+    other versions of Unity, but ``2020.3.12f1 (LTS)`` is the version the
+    SDK team uses for testing and development. We recommend using
+    this version to ensure your project works with the .NET SDK and that
     the install steps match the :doc:`Integrate Realm with Unity
     </sdk/dotnet/unity>` documentation steps because Unity's UI
-    often changes between versions. 
+    often changes between versions.
 
 
 Android Deployment
 ------------------
-Due to some instruction set limitations, Realm does not support 
-deploying Android apps to the ``armeabi`` ABI. Because default templates often 
-have different ABI settings for Debug and Release modes, your app may throw 
-a ``System.TypeInitializationException`` exception in Release mode but not when 
+
+Due to some instruction set limitations, the SDK does not support
+deploying Android apps to the ``armeabi`` ABI. Because default templates often
+have different ABI settings for Debug and Release modes, your app may throw
+a ``System.TypeInitializationException`` exception in Release mode but not when
 it was running in Debug mode.
 
-To avoid this, verify the ABI settings for both Debug and Release modes. To check 
-and change the settings, follow the steps in the 
-`Visual Studio CPU Architectures <https://docs.microsoft.com/en-us/xamarin/android/app-fundamentals/cpu-architectures>`_ 
+To avoid this, verify the ABI settings for both Debug and Release modes. To check
+and change the settings, follow the steps in the
+`Visual Studio CPU Architectures <https://docs.microsoft.com/en-us/xamarin/android/app-fundamentals/cpu-architectures>`_
 page.
 
-Unless you have a good reason to avoid linking other ABIs, we recommend 
+Unless you have a good reason to avoid linking other ABIs, we recommend
 checking all of the settings other than ``armeabi``.
 
-Limitations 
+Limitations
 ------------
-Realm has limits imposed to balance flexibility with performance. The SDK 
-throws an exception during app initialization if the following limits are 
+
+The SDK has limits imposed to balance flexibility with performance. The SDK
+throws an exception during app initialization if the following limits are
 exceeded:
 
-- Class names can't exceed 57 bytes in length. 
-- Property names can't exceed 63 bytes in length. 
+- Class names can't exceed 57 bytes in length.
+- Property names can't exceed 63 bytes in length.
 
-In addition, for iOS apps, the total size of all open Realm files cannot be 
-larger than the amount of memory your application is allowed to map in iOS. This 
-varies per device, and depends on how fragmented the memory space on the device is. 
-If you need to store more data than is allowed, you can split your data into 
-multiple Realm files, open a realm only when needed, and close it when it is 
+In addition, for iOS apps, the total size of all open Realm files cannot be
+larger than the amount of memory your application is allowed to map in iOS. This
+varies per device, and depends on how fragmented the memory space on the device is.
+If you need to store more data than is allowed, you can split your data into
+multiple Realm files, open a realm only when needed, and close it when it is
 no longer needed.
 
 For more information, see `Open Radar 17119975 <http://www.openradar.me/17119975>`_.