Update documentation CI setup to make sure documentation pages are build

[stijnvanhoey]

I'll add some more content on the package functions, the services and the architecture, but this PR makes the sphinx website working (see https://enram.github.io/vptstools/) and contains the docstring extractions , e.g. https://enram.github.io/vptstools/api/vptstools.html#vptstools.vpts.vpts