task: Switch to properdocs (from mkdocs)

[tuxuser]

see mkdocs dilemma: https://fpgmaas.com/blog/collapse-of-mkdocs/
and: https://squidfunk.github.io/mkdocs-material/blog/2026/02/18/mkdocs-2.0/

Refinements along the way:

• updated docs
• docker / compose
• CI workflows

[InvoxiPlayGames]

LGTM