Move on from build_sphinx

[lilyminium]

Problem

I think sphinx and setuptools integration is gone from 7 onwards -- see failures in #4276

I didn't think sphinx_rtd_theme supported 7.0 anyway but they should do from the next release, so let's fix this now than later.

[IAlibay]

Yeah it's a thing across the entire org I think.

[IAlibay]

Switching this to sphinx-build will probably do the trick?

mdanalysis/.github/actions/build-src/action.yaml

Lines 87 to 91 in 8d20cdc

	- name: build_docs
	if: ${{ inputs.build-docs == 'true' }}
	shell: bash -l {0}
	run: |
	cd package && python setup.py build_sphinx -E --keep-going