Integrate scm-backup-docs into this repo

[christianspecht]

Maintaining two separate repos doesn't make sense.

I'm making changes to the code and the respective documentation together all the time, and this would be much easier if it just was in the same branch of the same repo.

Read the Docs also supports getting the documentation from a doc subdir in the main repo.

[christianspecht]

On the other hand, the docs repo has only tags for minor versions (1.3, 1.4...) by purpose, because I don't want a separate docs version for every patch version.

Read the Docs automatically creates a version for each tag, but apparently I'm able to deactivate single versions at https://readthedocs.org/projects/scm-backup-docs/versions/. So I could just deactivate major.minor.patch versions as a "part" of the publishing process.