Update ReadTheDocs to use latest MkDocs

[d0ugal]

This is really an upstream issue, but I want to track it here for downstream users.

[d0ugal]

readthedocs.org is currently running 0.11.1 this means that they are missing all of these lovely commits which is roughly half of the MkDocs git history (based on number of commits).

[d0ugal]

After I roll a 0.13.0 release in the next few days I will be working on updating readthedocs to use it, currently there are problems due to the theme customisations they make.

[ericholscher]

👍 Happy to merge & deploy the new version when it's ready.

[d0ugal]

@ericholscher Thanks! Can we define what blocks are needed from themes so they work on ReadTheDocs? See #355 and readthedocs/readthedocs.org#978 (comment). I want to add blocks and reduce the duplication of the theme that you guys have - this should then reduce the risk of us breaking something if you only inject into small defined areas. We can then document that and add support in other themes.

[d0ugal]

See readthedocs/readthedocs.org#1305

[d0ugal]

This just landed in ReadTheDocs! w00t!