Summary of changes:
* Add a test for trailing whitespace, remove all trailling whitespace
* Add a test for broken URLs in docs, fix all broken URLs and permanent redirects
* Switch sphinx doc build to strict (warnings to errors) and nitpicky (broken links to warnings). Fix all sphinx warnings.
* Deploy docs from any commit to [kwant/kwant](https://gitlab.kwant-project.org/kwant/kwant) to https://test.kwant-project.org/doc/dev
* Deploy docs from any commit to master at kwant/kwant to https://kwant-project.org/doc/dev
* Deploy docs from any tag release tag (so anything that is vX.Y.Z) at kwant/kwant to https://kwant-project.org/doc/X.Y

I try to cleanly separate between information that should be part of the
Kwant source distribution (that is tied to a particular version of
Kwant), and generic information that is best presented on the website.

The pip tool messes up the timestamps of files, so with our previous strategy
of enabling cython by default the build fails if Cython is not available.

Also, turn the "warning" into a "caution" message that is less frightening and
send it to stdout only, so that it does not appear with pip.