chore: Refactor gazelle docs

[dougthor42]

The Gazelle docs are fully contained within gazelle/README.md. The docs are getting larger and this is starting to become harder to maintain and reference.

I'd like to see the docs added to the readthedocs page, with some logical section splitting.

I propose a structure:

./docs/
  ~~gazelle.md~~     # removed; TOC tree in index.md references gazelle/docs/index.md
./gazelle/
  docs/              # new folder
    index.md         # basic configuration, usage
    directives.md    # Details on # gazelle:foo comments found in BUILD.bazel files
    annotations.md   # Details on # gazelle:foo comments found in python source files
    development.md   # source code layout, how to add/run tests, bump versions, write docs, etc.
  README.md          # Most content moved to docs/*.md files; what's left is basically a stub

[dougthor42]

@aignas @rickeylev Which would you prefer to review?

One big PR that migrates everything at once?

Or a 4 PRs?

1. Core changes like BUILD files and the high-level Gazelle info (index).
2. annotations
3. directives
4. development