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

[dougthor42]

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.

[aignas]

LGTM

[aignas]

nit: have you considered using definition lists here?

I think typographically this would look better?

[dougthor42]

I'll look into it 👍 #3142.