Move documentation to readthedocs.org

Move the GWCelery documentation from GitLab Pages at the URL https://emfollow.docs.ligo.org/gwcelery/ to Readthedocs at the URL http://gwcelery.readthedocs.io/.

Readthedocs is the de facto standard solution for building and hosting documentation for open-source Python projects. Hosting the documentation on readthedocs.org comes with several advantages:

• The documentation features a dropdown menu to select among different versions and releases.
• Documentation is automatically built in not only HTML but also epub and PDF format.
• The documentation is a little bit easier to find, since Readthedocs is often the first stop for people looking for documentation for Python projects.
• Page load times are faster.

The docs will still be built by GitLab CI, although only to check that they build without warnings or errors.