docs: Add links to sidebar and frontpage

This adds links to destinations like the mailing list, the source code
browser, the code review tool, the chat room, and more.

Change-Id: I402cc98ce411dc37c2d0db6f06b62240cbafe931
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/14640
Reviewed-by: Armando Montanez <amontanez@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Keir Mierle <keir@google.com>
diff --git a/docs/index.rst b/docs/index.rst
index 804f86a..d7eff04 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -10,11 +10,16 @@
 
   Home <self>
   docs/getting_started.md
-  CODE_OF_CONDUCT.md
+  Source Code <https://pigweed.googlesource.com/pigweed/pigweed/+/refs/heads/master>
+  Code Reviews <https://pigweed-review.googlesource.com>
+  Mailing List <https://groups.google.com/forum/#!forum/pigweed>
+  Chat Room <https://discord.gg/M9NSeTA>
+  Issue Tracker <https://bugs.chromium.org/p/pigweed/issues/list>
   CONTRIBUTING.md
+  CODE_OF_CONDUCT.md
   docs/embedded_cpp_guide
-  docs/style_guide
+  Code Style <docs/style_guide>
   targets
-  build_system
+  Build System <build_system>
   docs/module_structure
   module_guides