download: Move content into per-OS tabs

Depends on !104 (merged) (security page in markdown, whereas this changes its renderer)

Might need some content adjustments, but looks roughly fine to me. Tab order etc. is of course subject to change (currently first/default is Linux, idk if that makes sense), looking for feedback.

download: Move content into per-OS tabs 

Adds a very simple tab system to the Download section.
I tried my best to make them simple and accessible - friendly for keyboard nav
(they're focusable by TAB, selectable by ENTER) and hopefully for screen readers as well.

To add some colour, tabs use our regular r/g/b scheme for selection indication - desktop OS's get red,
mobile ones get green, and the sources tab gets blue. :smile: 

www: Move from markdown to cmark-gfm
Needed mainly to allow parsing markdown inside some basic HTML block tags (the old markdown tool does not allow that).
Also allows for some useful extensions, like markdown tables, strikethrough etc.

Before	After