Check documentation style

[timokau]

We have recently added some static analysis and formatting tools. One thing we are not checking for yet is inline documentation. There are some tools out there, for example pycodestyle. It seems like pylint has some doc checking functionality too.

I think it would be valuable to extend our static checks to the inline documentation. We already check stand-alone rst files with doc8..

[kiudee]

Now that we have tox running a test build of the documentation, we can think about letting it fail the build, when warnings occur. We should of course fix the warnings first.