Fix UG/KB interactive functions

Front-end

• [ ] Clean up templates
• [ ] Clean up CSS (mostly done)
• [ ] Clean up JS
• [ ] KB submit/edit/manage pages

Back-end:

• [ ] https://www.phpbb.com/support/docs/ug/ and https://www.phpbb.com/support/docs/kb/ are dangerous, should probably redirect
• [ ] documentation_update.php needs updating (slugs used inside articles don't mach actual routes. They still use .php extensions etc)
• [ ] KB attachment images are often loaded over insecure http
• [ ] UG comments system is broken
• [x] UG side-menu items should scroll to `id="docbook"` when clicked. Otherwise mobile users have to scroll by the menu every time
• [ ] KB admin system is broken
• [ ] Old UG and KB links need to redirect to the new support/docs, especially those which are used in the community forums
• [ ] Optional: 3.0/3.1 toggle switch should link to the same UG page, but different version
• [ ] Optional: simplify the repositories, perhaps we can get rid of `docs-system` repo entirely?
• [ ] Optional: add support/docs/en/latest/kb/ so we can always link to the latest version?

When everything is finished:

• [ ] Remove documentation.css
• [ ] Remove all traces of the old docs/UG/KB systems