message: doc: add g3doc sitemap to toplevel readme
  No-Try: true
  Cr-Commit-Position: refs/heads/master@{#34538}
diff: @@ -27,5 +27,6 @@ native API header files.
  * [Coding style guide](g3doc/style-guide.md)
  * [Code of conduct](CODE_OF_CONDUCT.md)
  * [Reporting bugs](docs/bug-reporting.md)
  + * [Documentation](g3doc/sitemap.md)
  [native-dev]: https://webrtc.googlesource.com/src/+/main/docs/native-code/index.md
  