Turn wiki into docs/ of Markdown

[olleolleolle]

This Issue is about turning the wiki into git-controlled Markdown files.

it’s easier to maintain and keep in track with the code. It also allow you to jump into a sort of “time machine” by checking out any past tag and seeing how the doc was at that point in time, which I find really useful

Also, updates to the documentation can be part of the same PR where new features are introduced or the API is changed

This is how Faraday has it: https://github.com/lostisland/faraday/tree/master/docs

In order to start

• [x] find the relevant pages
• [x] turn each relevant page into a Markdown document in docs/
• [x] close the wiki for editing (if possible)
• [x] link to each in-repo document from its wiki page. This has been done on the frontpage, at least.

[olleolleolle]

This is the setting we now use.

[olleolleolle]

Added the note about Markdown documents in repo to the sidebar in the wiki: