Help page: add multi-level TOC, document "Again", misc.

Added

• Add "Help" link to the desktop navbar by @fabd in #237
  • Use a "More" label in the desktop nav instead of just a hamburger icon
  • Add link to Github Discussions in the "More" dropdown of the deskop nav
• Improve documentation, add multi level TOC, ... by @fabd in #235

Changed

• Refactor : add foolproof checks for SRS ratings and database updates by @fabd in #233

Fixed

• My Stories list duplicate rows when paging by @fabd in #231
• Remove link to obsolete forum from the mobile nav by @fabd in #236