Docs toolchain

Scope: Python projects (Home Assistant components, updates2mqtt, CLIs).

• Documentation uses properdocs with the materialx theme. No mkdocs.yml.
• New projects with simple needs may use zensical as a trailblazer, until it supports the plugins the other projects rely on.
• Non-Python projects use astro with starlight, to avoid adding dependencies.

Exceptions: plugin packages keep their mkdocs- names.

Background: the migration project is in projects/done/mkdocs_decom.md.