Update the GitLab Pages documentation

[walpox]

Sanity check:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Rationale

1. What's stated in the section Repository setup seems to be outdated. As per the current documentation on GitLab, GitLab Pages does not seem to work the way described there.
2. In the GitLab CI template, the only keyword is no longer supported by GitLab. Its replacement is rules.
3. The GitLab CI template can be changed to allow using specific versions of Zola.
4. Rewritten some parts as per my personal writing style.
5. Some other changes were proposed by my Markdown linter, such as removing the dollar signs from the fenced code blocks.
6. Removed comments that described what a command does, instead of the why.

[Keats]

Thanks