Move docs to readthedocs

[mtazzari]

Readthedocs has many advantages, including automatic versioning of documentation.
Our current process to produce the docs is with the doctr tool which relies on travis. Since in #184 we are moving away from Travis, we need to replace the docs build. readthedocs.org is the natural place where to move the docs.

Note the successful migration for https://github.com/mtazzari/uvplot in PR mtazzari/uvplot#47