Move docs to MKDocs

[timlinux]

To be consistent with our other doc work and to make the docs easier to contribute to, I want to suggest to switch our docs over to mkdocs, with a github autobuild.

You can find a worked example here:

https://github.com/timlinux/QGISAnimationWorkbench/blob/main/.github/workflows/mkdocs.yml https://github.com/timlinux/QGISAnimationWorkbench/blob/main/mkdocs.yml https://github.com/timlinux/QGISAnimationWorkbench/tree/main/docs

and the published artifacts get pushed to here:

https://github.com/timlinux/QGISAnimationWorkbench/tree/gh-pages

And published here:

https://timlinux.github.io/QGISAnimationWorkbench/