Integrate documentation in main branch

[realrolfje]

The site at realrolfje.github.io/anonimatron is currently rendered from the gh-pages branch. With the new options provided by GitHub to render from another folder in the main branch, it would be nicer to have the site render from a documentation folder the same branch the sources are in.

This would also enable developers to document the usage of Anonimatron and new features alongside their edits on the code.

It would synchronize the release times (merge to master) of the documentation and the sources: the website renders from the master branch.