Migrate Onionbalance documentation to Onion MkDocs

Description

Migrate Onionbalance documentation from Sphinx to Onion MkDocs.

Tasks

• Initial conversion.
• Fix logo (done at tpo/web/onion-mkdocs!20 (merged)).
• Setup redirections for the old, original HTML pages (using HTML redirections, which don't depend on the underlying HTTP server).
• Review and cleanup.
• Rename and restructure documents (use the redirects MkDocs plugin to keep former URLs working).
• Review again:
  • ChangeLog: link to issues. Done at !23 (merged).
• Update the mirrors:
  • Disable building in the mirrors (to the new canonical URL).
  • Apply redirects in the mirrors:
    • Mirror 1.
    • Mirror 2.