Import ubuntu-touch-sdk-api-scripts to Focal & migrate to GitLab

https://github.com/ubports/ubuntu-touch-sdk-api-scripts Archived

xenial should become ubports/xenial with main branched from that becoming the new default; the current default master branch appears to be out-of-date with only a few dependency version bumps merged. Done

@peat-psuwit also mentioned on UBports Development telegram this being manually configured in CI (https://ci.ubports.com/job/api-docs-source) and that:

Its config seem to point into a script I can't find in the repo. So, you probably have to look into that if it hardcodes the repo name or something