Setup GitLab CI to build docs

Build instructions on https://gitlab.com/ubports/infrastructure/ubuntu-touch-sdk-api-scripts/-/blob/main/README.md, needs vagrant and virtualbox to build docs in the VM.

I'm wondering what should be done with the Xenial branch, do we still want to provide the old docs under another subdomain for example? Would be nice to have for diffing them somehow due to the renames of a bunch of stuff to Lomiri from Ubuntu etc.

At the end of the process there should be deployment via e.g. GitLab pages, basically integrate https://github.com/ubports/api-docs into this repo if possible.