feat: keep CI built documentation as an artefact

For the moment we use readthedocs.org to host the guidebook.

We may move to GitLab pages in the fullness of time and so pave the way by keeping the documentation built by the test job around. This build artefact is also linked to in the MR description:

This has the follow-on advantage that one can now browse the docs for any branch straight from the CI job page.

E.g., the linked artefacts on this MR ultimately take you to https://uis.uniofcam.dev/-/devops/docs/guidebook/-/jobs/1025744/artifacts/public/index.html.