Publish master docs with gitlab pages.
Relevant to #212 (closed).
This only takes care of the docs on the master branch. Ebassi proposed a possible solution for all branches but I won't have any spare time to work on it sadly.
A more detailed description can be found in the issue #212 (closed), as mentioned above.
After the merge the docs links will be the following:
https://gnome.pages.gitlab.gnome.org/gtk/gtk/