mdBook improvements

[juliangiebel]

This PR Reimplements the theme files to get rid of some cruft and makes it work with the latest version of mdBook (fixes the issue with links).
This also means the improvements to the default theme from the new mdBook versions are in our theme now.
Additionally the navigational arrows are restored (why where they removed in the first place, in addition to that breaking in the new versions) and some other bugs where fixed smaller improvements were made.

Some of the improvements

• The navigation items only expand by default up to the first level now to declutter the navigation sidebar.
• The text body is allowed to take up more horizontal space
• The navigation arrows on the sides of the page to easily go between pages are back also improving mobile navigation
• The broken TOC generator was removed (It's abandoned and doesn't work with the latest version of mdbook)

Stuff that got removed

• The generated TOC on the right side

This PR also includes a VSCode profile for mdbook because setting up a dev environment for it is a pita.

Here is an instance that's using this PR: https://docs.ss14.dev