Fix building mkdocs

[dklimpel]

Checklist

• make test-all (UNIX) passes. CI will also test this
• unit and/or integration tests are included (if applicable)
• documentation is changed or added (if applicable)

Description of change

Fix docs pipeline. Build and deploy docs to branch gh_pages. It has do be enabled in https://github.com/goss-org/goss/settings/pages

[aelsabbahy]

Thank you for opening up this PR, looks promising.

There were some discussions here on readthedocs vs gh-pages: #856

@noirbizarre any idea why the docs build currently fails?

@dklimpel , let's give @noirbizarre a chance to provide feedback. If we don't get any, I can approve and switch to this approach.

Personal bias, I'm looking for the easiest thing to support.

[dklimpel]

Thank you for your feedback.

I didn't realise that the decision had been made in favour of readthedocs.

The reason for the failing CI is:

Error: No file in /home/runner/work/goss/goss matched to [**/requirements.txt or **/pyproject.toml], make sure you have checked out the target repository

The setup python job needs for caching a requirements.txt and not an requirements.pip file.