docs: friendlier user guide with screenshots

Rewrite the user guide with a warm, step-by-step tone, Material admonitions
(tips/notes/warning) and inline screenshots. Add docs/img/ (downscaled captures:
library, inbox, player+chapters, detail, queue, settings, iPad) and a styles.css
for polished rounded screenshots. Enable attr_list/md_in_html. MkDocs builds clean.

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