Move documentation to the `bootstrap` repo, but generate it on the website too

As discussed in https://code.greenhost.net/openappstack/bootstrap/merge_requests/44 we would like to keep documentation close to the code. At the same time we want to display that documentation on the website.

Two scenarios have been proposed:

1. The whole website lives in a subfolder in this repo. That way all documentation is close to the code (and up to date with a specific branch)
2. The website keeps living in the /website repo, but pulls documentation files from this repo (could be a subfolder in this repo with documentation *.md files that are rendered on the website and links are generated dynamically on https://openappstack.net/documentation.html