docs: use rst-roles
Sphinx / RST has a lot of predefined roles that generally helps making the docs a bit more semantically strong. The precise benefits depend a bit from role to role, but here's some usual benefits:
- Rendering: Making things render in a consistent way.
- Navigation: Some of these (like the
:envvar:
-role) supports cross-linking between definitions and references. See !19593 (merged) for more details in enabling this. - Spell-checking: Makes it clear what is normal language, and what is text with special meaning.