Syntax highlight API Blueprint in documentation

[kylef]

This PR uses pygments-apiblueprint to add syntax highlighting for API Blueprint to the rendered documentation on readthedocs. While pygments-apiblueprint is not perfect, it is certainly better than no syntax highlighting. If you spot any rendering issues with API Blueprints feel free to open issues over there and I'll try fixing (I did find kylef/pygments-apiblueprint#1 which I'll fix but no need to block this PR).

Note, this also version locks all of the mkdocs/pygments and related dependencies so that they are not changed and break the docs. If you'd rather not lock the versions I can change requirements.txt to not specify versions, but I think it would be a good idea to lock them so documentation doesn't silently break when a version of mkdocs or dependency has a breaking change.

For readthedocs to install the dependencies declared in docs/requirements.txt the path must be specified on read the docs admin. Please enter it there before merging:

Before

After

[honzajavorek]

Thanks! Let's see how the result of the build will look like 👍

[honzajavorek]

Looks great!