Move mkdocs action/workflow into `docs` group

[jayceslesar]

Feature Request / Improvement

I was trying to set up mkdocs on this repo and found the existing setup a little unintuitive -- in most cases I use mkdocs as a docs extra in whatever dependency manager I am using (PDM or Poetry).

I can work on adding this as an extra so there isnt some extra hidden workflow in requirements.txt inside of an mkdocs folder