Improve GitLab CI pipeline to match provided GitHub CI as closely as possible

[apirogov]

Currently, the Gitlab CI is only rudimentary and lacks behind the Github CI scripts.

Before implementing #11 the Gitlab pipelines should be brought to the same or similar level.

Maintenance of both sets of scripts should be as easy as possible, thus keeping them similar would help.

Ideally, the Gitlab CI is also run from this repo in a Gitlab Demo repo to verify that everything works and to make Gitlab a first-class target.

• [x] support Gitlab Pages for docs
• [x] support Gitlab Releases
• [x] support PyPI Release from Gitlab (must use TOKEN-based)
• [ ] update docs accordingly

[apirogov]

The API of Github and Gitlab on URL level is apparently compatible, i.e. given a repo base URL, there is no need to change construction of

• Issue tracker endpoint (/issues)
• Tagged commit endpoint (/tree/vX.Y.Z)
• Markdown edit page endpoint/fragment (/edit/main/docs or /blob/main/docs)

Need further to understand the URL logic for gitlab pages (probably also very similar)

For the direct cloning link the syntax must be: git+ssh://git@domain.com:path/to/repo.git so we need to split the project URL to rewrite it like that.

The major issue is how to avoid complication in user prompting. Current idea:

• Ask user via cookiecutter to provide the URL of an empty repository (where the repo will be pushed to)
• infer all remaining variables from it, i.e.
  • domain (github.com / some.gitlab.org)
  • project path (orga[/suborga])
  • possibly the project slug (repo name at end of path)