docs: Add docs about creating a new SDK (#12560)

This adds a doc page with infos about creating a new SDK.

See:
https://github.com/getsentry/sentry-javascript/blob/fn/new-sdk-docs/docs/creating-a-new-sdk.md

---------

Co-authored-by: Luca Forstner <luca.forstner@sentry.io>
Co-authored-by: Andrei <168741329+andreiborza@users.noreply.github.com>

Author: 3 people

docs/creating-a-new-sdk.md

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+# Creating a new SDK
+
+While each SDK (e.g. `@sentry/react` or `@sentry/nextjs`) is somewhat unique, we try to follow some general themes when
+creating a new SDK.
+
+## Types of SDKs
+
+Broadly speaking, there are three types of SDKs:
+
+1. Browser SDKs (e.g. `@sentry/react` or `@sentry/angular`)
+2. Server SDKs (e.g. `@sentry/bun` or `@sentry/node`)
+3. Meta SDKs (e.g. `@sentry/nextjs` or `@sentry/sveltekit`) - which cover both Browser & Server
+
+Depending on what type of SDK you are creating, you'll have to include different things.
+
+## General Guidelines
+
+As a rule of thumb, we should follow these two ideas:
+
+1. Whenever possible, instrumentation should work without (or with as little as possible) user configuration.
+2. Instrumentation should follow common patterns for a specific platform. No config is always preferred, but if config
+   is unavoidable, it should feel as native as possible to users of the given framework.
+
+## 1. Browser SDKs
+
+A purely browser SDK generally should cover the following things:
+
+### 1a. Error Monitoring
+
+We have global error handlers out of the box. However, in many frameworks there are ways for users to capture errors
+too, which may lead to them not bubble up to our global handlers. Generally, the goal is that all errors are captured by
+Sentry.
+
+Either we should use some hook (e.g. `app.on('error')`) to capture exceptions, or provide composables (e.g. an error
+boundary component in React) that users can use in their app to ensure all errors are captured by Sentry.
+
+### 1b. Performance Monitoring
+
+#### Routing Instrumentation
+
+At a minimum, each browser SDK should have **Routing Instrumentation**.
+
+While we have a default `browserTracingIntegration`, this has no access to a router, and is thus only based on URLs. We
+should strive to provide a custom `browserTracingIntegration` for SDKs that can leverage the router & routing
+information.
+
+Ideally, this means that we can emit pageload & navigation spans with parametrized route names instead of full URLs.
+
+Some of the following concepts may be relevant to your SDK:
+
+- **Redirects**: If possible, we want to skip redirects. This means that if a user navigates to `/`, and this redirects
+  the user internally to `/dashboard`, we only want to capture a single `/` navigation/pageload.
+- **Route Params**: Routes should be parametrized, which means that instead of `/users/123` we want to capture
+  `/users/:id` or simmilar.
+- **Query Params**: Query params should generally be removed from the route.
+
+#### Component Tracking
+
+Additionally, depending on the framework we may also have **Component Tracking**. We may track the duration of component
+renders and similar things. These are stretch goals, though, and do not need to be part of an MVP.
+
+## 2. Server SDKs
+
+A purely server SDK generally should cover the following things:
+
+### 2a. Error Monitoring
+
+We have global error handlers out of the box. However, in many frameworks there are ways for users to capture errors
+too, which may lead to them not bubbling up to our global handlers. Generally, the goal is that all errors are captured
+by Sentry.
+
+Either we should use some hook (e.g. `app.on('error')`) to capture exceptions, or provide composables (e.g.
+`setupFastifyErrorHandler(app)`) that user can call.
+
+### 2b. Performance Monitoring
+
+#### Routing Instrumentation
+
+At a minimum, each Node SDK should have **Routing Instrumentation**.
+
+Most SDKs that build on top of `@sentry/node` should automatically have basic `http.server` spans emitted for incoming
+requests by the `httpIntegration`. However, these spans do not contain any routing information (e.g. a `http.route`
+attribute). A server SDK should make sure to add route information to these spans.
+
+If there are things that should be captured in spans that are not covered by `httpIntegration`, we may need to write our
+own instrumentation to capture `http.server` spans.
+
+Some of the following concepts may be relevant to your SDK:
+
+- **Route Params**: Routes should be parametrized, which means that instead of `/users/123` we want to capture
+  `/users/:id` or simmilar.
+- **Query Params**: Query params should generally be removed from the route.
+
+#### Middleware Tracking
+
+Additionally, Node SDKs may also do **Middleware Tracking**. If possible, we may want to instrument middlewares, and
+create spans for them. These are stretch goals, though, and do not need to be part of an MVP.
+
+### 2c. OPTIONAL: Additional features
+
+We may also want to instrument additional features, if applicable, including:
+
+- Automatic cron instrumentation
+- [Cache module](https://docs.sentry.io/product/insights/caches/) - See
+  [Instrument Caches](https://docs.sentry.io/platforms/javascript/guides/connect/tracing/instrumentation/custom-instrumentation/caches-module/)
+- [Queue module](https://docs.sentry.io/product/insights/queue-monitoring/) - See
+  [Instrument Queues](https://docs.sentry.io/platforms/javascript/guides/connect/tracing/instrumentation/custom-instrumentation/queues-module/)
+
+## 3. Meta SDKs
+
+Meta SDKs should contain both the things pointed out in 1. and 2, _PLUS_:
+
+### 3a. Connected Traces
+
+Traces from SSR (server side) should be continued in the client side (browser). Usually this means that we have to
+inject the trace data as `<meta>` tags into the rendered HTML pages. If possible, we should do that automatically. If
+there is no way to do that automatically, we should provide a utility for users to do it themselves.
+
+### 3b. Instrumented Server Components / API Routes / etc.
+
+Depending on the framework, we should instrument all the pieces that exist in this framework. This includes capturing
+errors & spans for things like:
+
+- Server Components
+- API Routes
+- Layouts
+- etc.
+
+When possible, we should auto-capture this. If not possible, we should provide utilities for users to do this
+themselves.
+
+### 3c. Bundler Integration / Source Maps
+
+When possible, Meta SDKs should integrate with the used bundler. For example, SvelteKit uses Vite, so we should
+automatically set up `@sentry/vite-plugin` for the user. At a minimum, we want to enable source maps upload for the meta
+SDK, but this may also include automated release creation and other bundler features.
+
+We _should not_ expose the bundler plugin config directly, because this means that we cannot bump the underlying bundler
+plugin version in a major way (because the bundler plugin config becomes public API of the meta SDK). Instead, we should
+provide an abstraction layer of options that we expose on top of that.
+
+### 3d. Alternate JS Runtimes
+
+We generally want to support Node runtimes for the server. However, sometimes there may be alternate runtimes that may
+be supported, e.g. Cloudflare Workers or Vercel Edge Functions. We generally do not need to support these in an MVP, but
+may decide to support them later.