Configure Gitlab CI for tests and docs
Adds a CI config which does this after each push (any branch/tag):
- run
make test
- build sphinx docs and save them to gitlab's artifacts, so they can be downloaded/browsed from gitlab's ui (they expire in a week by default, but it's configurable, example for this branch)
- (for tags only) publish the built docs to gitlab pages at https://labs.pages.labs.nic.cz/yangson (returns 404 until this is merged)