[Documentation]: Robust documentation builds

[edwardchalstrey1]

Requested documentation improvement

Following discussion in #540 it might be worth reviewing and/or simplifying the build process so contributors can check any edits before merging them and also ensure that no bugs are introduced:

• Add version numbers for the packages used in doc/requirements.txt
• Ensure local build of Sphinx and any other docs requirements/ environment match what is being done by Readthedocs for consistency
• Ensure that the .. ipython:: parts of the RST docs work correctly
  • Do we need to add iPython to doc/requirements.txt? but how did this work before?
• Add PR preview feature in Readthedocs
• Other GH actions should not run when only docs files edited

Pre-requisite

#541