Add a first pass at some standard architecture diagrams for our typical
setups which can be re-used in documents intended for an external
audience. For the moment, lay the ground with an overview and ones
focussing on static IP-based ingress and egress. We can add more as need
arises.

Closes #215

 Closes: https://gitlab.developers.cam.ac.uk/uis/devops/docs/guidebook/-/issues/269

Move the labels and review discussion out of the workflow reference and
into appropriate reference and explanation guides.

Add in the ability to render GitLab labels as they appear on GitLab.
This provides greater visual similarity between GitLab and the
documentation.

Perform a minimal conversion of the Guidebook over to using mkdocs for
rendering.