Add extension-types to sidenav for release (dart-lang#5550)

Edit: Also fixes prev/next on surrounding pages.

Merge before the release

Author: MaryaBelanger

src/_data/side-nav.yml

@@ -82,6 +82,8 @@
           permalink: /language/enums
         - title: Extension methods
           permalink: /language/extension-methods
+        - title: Extension types
+          permalink: /language/extension-types
         - title: Callable objects
           permalink: /language/callable-objects
     - title: Class modifiers

src/content/language/callable-objects.md

@@ -4,8 +4,8 @@ description: Learn how to create and use callable objects in Dart.
 js: [{url: 'https://dartpad.dev/inject_embed.dart.js', defer: true}]
 toc: false
 prevpage:
-  url: /language/extension-methods
-  title: Extension methods
+  url: /language/extension-types
+  title: Extension types
 nextpage:
   url: /language/class-modifiers
   title: Class modifiers

src/content/language/extension-methods.md

@@ -5,8 +5,8 @@ prevpage:
   url: /language/enums
   title: Enums
 nextpage:
-  url: /language/callable-objects
-  title: Callable objects
+  url: /language/extension-types
+  title: Extension types
 ---
 
 Extension methods add functionality to existing libraries.