Move toolchains and execution group guides from "Design Docs" to "Extending"

RELNOTES: None
PiperOrigin-RevId: 532559956
Change-Id: I5b0b22e3c56d65c3a2ce00e34065537f60cc97f9

Author: aiuto

site/en/_book.yaml

@@ -272,8 +272,12 @@ upper_tabs:
         path: /extending/repo
       - title: Configuring
         path: /extending/config
-      - title: Platforms and toolchains
+      - title: Platforms
         path: /extending/platforms
+      - title: Toolchains
+        path: /extending/toolchains
+      - title: Execution groups
+        path: /extending/exec-groups
     - name: Writing rules
       contents:
       - title: Creating a rule
@@ -309,12 +313,6 @@ upper_tabs:
       - title: Document rules with Stardoc
         path: https://github.com/bazelbuild/stardoc
         status: external
-    - name: Design docs
-      contents:
-      - title: Execution groups
-        path: /extending/exec-groups
-      - title: Toolchains
-        path: /extending/toolchains
     - name: APIs
       contents:
       - title: Build file API