[doc] Move docs to github pages

This PR does the last things pending for migrating the doc build and hosting from readthedocs to github pages.

See https://github.com/taurus-org/taurus/issues/408#issuecomment-329782603

Basically it eliminates the mocks and it adds a link to other versions of the docs.

Note: with this migration we will lose the downloadable pdf docs, but IMHO it can be added later on if someone is interested on it.

Closes #408 (closed)