Restrict GitHub Pages building to PRs on main branch

[ns-rse]

Currently our documentation isn't available and I think it is because the .github/workflows/sphinx_docs_to_gh_pages.yaml runs on any push to any branch it seems.

We should restrict this to only run on pulls to the main branch.

Switch the following...

on:
  push:
    # Uncomment if only tagged releases are to have documentation built
    # tags:
    #   - v*
  workflow_dispatch: # Uncomment line if you also want to trigger action manually

...to...

on:
  pull_requests:
    branches:
      - main
  workflow_dispatch: