docs(gazelle): Migrate Gazelle docs to ReadTheDocs, part 2/5: installation and usage

[dougthor42]

Part of #3082

2nd of probably 5 PRs.

• Migrate installation and usage info from gazelle/README.md to gazelle/docs/installation_and_usage.md
• Slight rewording and reformatting of the Example section.
• Reorganized and modernized the MODULE.bazel and BUILD.bazel examples:
  • less details on rules_python as that's available in other docs
  • default to gazelle multilang support now that feat(gazelle): Gazelle plugin generates py_proto_library #3057 is merged.
• Mechanical updates:
  • Wrap at ~80 chars
  • Use MyST directives and roles.