Add redirections from old URLs to avoid breakages

The Scala 3 reference documentation that used to be published to https://docs.scala-lang.org was using some different URLs than the URLs used by the documentation produced by Scaladoc. You can see here some examples of breakages:
scala/docs.scala-lang@763d518

We add redirects from previously supported URLs to avoid such breakages in case there are links in the web that point to the old URLs.

Author: julienrf

docs/_docs/reference/changed-features/changed-features.md

@@ -2,6 +2,7 @@
 layout: index
 title: "Other Changed Features"
 movedTo: https://docs.scala-lang.org/scala3/reference/changed-features.html
+redirectFrom: /changed-features.html
 ---
 
 The following pages document the features that have changed in Scala 3, compared to Scala 2.

docs/_docs/reference/contextual/contextual.md

@@ -2,6 +2,7 @@
 layout: index
 title: "Contextual Abstractions"
 movedTo: https://docs.scala-lang.org/scala3/reference/contextual.html
+redirectFrom: /contextual.html
 ---
 
 ### Critique of the Status Quo

docs/_docs/reference/dropped-features/dropped-features.md

@@ -2,6 +2,7 @@
 layout: index
 title: "Dropped Features"
 movedTo: https://docs.scala-lang.org/scala3/reference/dropped-features.html
+redirectFrom: /dropped-features.html
 ---
 
 The following pages document the features of Scala 2 that have been dropped in Scala 3.

docs/_docs/reference/metaprogramming/metaprogramming.md

@@ -2,6 +2,7 @@
 layout: index
 title: "Metaprogramming"
 movedTo: https://docs.scala-lang.org/scala3/reference/metaprogramming.html
+redirectFrom: /metaprogramming.html
 ---
 
 The following pages introduce the redesign of metaprogramming in Scala. They

docs/_docs/reference/other-new-features/other-new-types.md renamed to docs/_docs/reference/other-new-features/other-new-features.md

@@ -2,6 +2,7 @@
 layout: index
 title: "Other New Features"
 movedTo: https://docs.scala-lang.org/scala3/reference/other-new-features.html
+redirectFrom: /other-new-features.html
 ---
 
 The following pages document new features of Scala 3.

docs/sidebar.yml

@@ -69,7 +69,7 @@ subsection:
           - page: reference/metaprogramming/reflection.md
           - page: reference/metaprogramming/tasty-inspect.md
       - title: Other New Features
-        index: reference/other-new-features/other-new-types.md
+        index: reference/other-new-features/other-new-features.md
         subsection:
           - page: reference/other-new-features/trait-parameters.md
           - page: reference/other-new-features/transparent-traits.md

project/resources/referenceReplacements/sidebar.yml

@@ -49,7 +49,7 @@ subsection:
       - page: reference/metaprogramming/reflection.md
       - page: reference/metaprogramming/tasty-inspect.md
   - title: Other New Features
-    index: reference/other-new-features/other-new-types.md
+    index: reference/other-new-features/other-new-features.md
     subsection:
       - page: reference/other-new-features/trait-parameters.md
       - page: reference/other-new-features/transparent-traits.md