revamp the contribution guide

This cleans up a small part of the contribution guide. Probably good to merge as is.

The missing stuff is mainly the VSCode+WebIDE contributions; we need to make new screenshots there, avoiding the develop and highlighting the "update card" link.

To discuss: reordering of the cards in the category.

EDIT: This cleans up almost the whole contribution guide. Decided to retain a lot of stuff, mainly docs for the gitlab flow. Good to go now AFAIK.

The other MR in common shall point the big contribute card on frontpage directly to the "overview" card here.