Migrate docs to its own repository (or gh-pages branch)

[magicDGS]

As the repository is growing bigger and with more versions, it will be nice to have a versioned help pages (see #423). This can be handle in a different repository or just in a different branch. Both ideas have their own pros/cons - any solution could be accompained for a newer and custom style, getting rid of the current jekyll-documentation-theme which does not allow folders.