Extend Gitlab CI/CD pipeline

[achim-w]

I splitted the pull requests for the GitLab CI/CD pipeline in 2 separate requests:

• The 1st one more closely follows the GitHub actions.
• This pull request further extends the GitLab CI/CD pipeline with additional features.

This way, the additional features could be easer backported to GitHub actions if you would like. The features introduced are:

• Use site_url: Currently, site_url in mkdocs.yml is not set, so the sitemap.xml in the rendered output is empty (see e.g. the output of the ToT). The site_url is now set by the GitLab pipeline, if no URL is specified in mkdocs.yml by the user. See also e.g. this stackoverflow discussion for more detail about site_url and sitemap.xml.
• Upload the rendered html site to Zenodo: Instead of only uploading the files of MkDocs, the pipeline also uploads the rendered html page to Zenodo, archiving also the output.
• Tag the commits of the versions uploaded to Zenodo: Since the "release tag" does not point to the version that is uploaded to Zenodo (since the metadata is updated by the pipeline before the upload), the pipeline now introduces additional tags, one for the MkDocs files (suffix -released-source and one for the rendered html pages files (suffix released-pages). This makes it easier to find the commits that have been uploaded to Zenodo.
• Fix URLs in linkset.json for items: The automatically discovered items for linkset.json currently all point to the main branch of the repository, which is wrong for released versions (see e.g. the 1.3.0 release of ToT: all the URLs under item point to the main branch of the repository.
• Additionally, I synced the metadata files to what they would be after the first run of the pipeline, that the first pipeline produces less noise in the first commit.

[achim-w]

@korvoj Thanks for the quick merge of the first pull request. How should we proceed with this one? Should I rebase it on current main that it is easier to review? Do you want to discuss anything for backport to GitHub actions or do you want to do this at a later time?