Consolidate documentation

[almet]

Currently, the documentation lives within markdown files on different parts of the repository.

This has the advantage of being simple, but makes it harder to search, and sometimes I have trouble finding the information I'm looking for.

Maybe we could have a sphinx / mkdocs tool in place which would compile all this documentation and make it available somewhere, maybe making it available at https://docs.dangerzone.rocks

I see the following advantages:

• The doc will be easier to search
• As part of this effort, we could create an index of our documentation
• We could add some documentation, for instance to help newcomers contribute to the project.