docs(gazelle): Migrate Gazelle docs to ReadTheDocs, part 4/5: directives (#3139)

Part of #3082

4th of probably 5 PRs.

+ Migrate directive docs from `gazelle/README.md` to
`gazelle/docs/directives.md`
+ Switch from table-based summary to bulleted lists
  + This will be much easier to maintain going forward.
+ Add dedicated sections for each directive
+ Though not filled out yet. I do plan on filling them out later, I just
can't say when.
+ Mechanical updates:
  + Wrap at ~80 chars
  + Use MyST directives and roles.

Author: dougthor42