[WEBSITE-1122] New Docs System

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The styling of the new docs system needs to be fixed (and optimized).

[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
[x] Support pages like https://www.phpbb.com/support/ need to link to the new support/docs (also for symfony website)
[x] 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
[x] 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] Links to other KB articles inside KB articles (linking from one KB to another) are broken
[x] Direct KB links with missing trailing slash are broken
[ ] KB attachment images are often loaded over insecure http
[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?
[ ] Optional: add support/docs/en/latest/kb/ so we can always link to the latest version?

_When everything is finished:_