Reorganize the links on the codebase docs index page.
Review Request #7895 — Created Jan. 18, 2016 and submitted
The codebase docs was growing in terms of content, but it was resulting in links to docs not having a great place to go. This change adds some new sections that we'll be able to fill up with links, which will help new contributors locate the docs that they want.
Built the docs and tested the links.
Tool: Pyflakes Ignored Files: docs/codebase/index.rst Tool: PEP8 Style Checker Ignored Files: docs/codebase/index.rst