WIP Refactor docs to be consistent with other kartoza projects

[timlinux]

Key improvements:

• move docs to their own folder rather than mixing in with the source tree
• nix-shell for a repeatable build env (HTML AND PDF)
• added pdf build workflow, templates etc.
• consistency with other kartoza projects

There are a few tweaks I need to make before we can merge:

• update GH workflow for publishing docs
• add some missing MKDocs plugins and directives

[github-actions]

Plugin zip package for the changes in this PR has been successfully built!.

Download the plugin zip file here https://github.com/kartoza/cplus-plugin/suites/19903545378/artifacts/1177899872

[Samweli]

@timlinux I have made several updates to solve issues with the CI tests failing, we now need to change the documentation actions workflow in order to be able to deploy the documentation site into the github pages.