Clean up README, improve documentation

[jayconrod]

• Translate existing documentation from reStructuredText to Markdown for more familiar editing. (Unfortunately, this makes tables much harder, so we'll reformat flags, attributes, and directives.
  • README.rst
  • Design.rst
  • autogazelle.rst
• Split README into multiple documents, loosely following https://diataxis.fr/.
  • README should have a brief "quick start" guide to getting started with Gazelle with examples for Bzlmod and WORKSPACE. It should link to other documents and other Gazelle extensions.
  • Reference documents: separate pages for CLI flags, the gazelle rule, and directives.
  • Go- and protobuf-specific reference documents should be separated too.
• Extending Gazelle should be reframed as a tutorial.
• Architecture of Gazelle should be rewritten and reframed as an explanation: it predates the extension mechanism.

[jayconrod]

Didn't mean to close this, not done.