Task/activate new docs

old/: https://github.com/phpbb/website/pull/74

https://tracker.phpbb.com/browse/WEBSITE-1122

- [x] Move all docs/ug/kb assets from `website.css` to a new `support_docs.css`
- [x] Only load `support_docs.css` on `support/docs/` pages (old ug/, kb/ and documentation/ pages wont work anymore)
- [x] Fix most glaring JS errors
- [x] Move `documentation.css` into `support_docs.css`
- [ ] Clean up templates
- [ ] Clean up CSS (mostly done)
- [X] Clean up JS
- [x] Make responsive
- [x] Make 3.0 / 3.1 toggle switch
- [x] 3.0 / 3.1 switch backend
- [ ] Comments backend
- [ ] Manage / admin pages
- [ ] Support pages like https://www.phpbb.com/support/ need to link to the new support/docs (also for symfony website)
- [ ] The main .com header navigation needs to link to the new support/docs (also for symfony website)
- [x] Fix UG ajax article loading
- [x] Have links for UG PDF downloads
- [x] support/docs welcome page needs some proper text copy
- [ ] Sync assets with symfony website
- [ ] Check all templates (both old/ and symfony) for references to old docs/ug/kb

__What we need for the backend:__
- [ ] 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)
- [x] Migrate all KB articles to the new docs system
- [x] `S_IN_DOCS_ASCRAEUS` or `S_IN_DOCS_OLYMPUS` need to always be defined when in support/docs
- [ ] 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 categories (can't test them properly at the moment)
- [ ] 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?

__When everything is finished:__
- [ ] Remove documentation.css
- [ ] Remove all traces of the old docs/UG/KB systems