docs: update wiki/Pages with all current features

Rewrite the user guide (inbox, search, tags/sort, sleep timer, chapters, notes &
share, OPML import/export, settings) and refresh the README/index feature lists,
ROADMAP (post-MVP done + next), CHANGELOG (chapters). Sync now documented under
Réglages. MkDocs site builds clean.

Co-Authored-By: Claude <claude@anthropic.com>