DOCS: Remove sphinx temporary maximum pin

[bjlittle]

📰 Custom Issue

Remove the temporary maximum pip pin sphinx<5 introduced into the setup.py by #5122

[tkknight]

Tested an unpin of sphinx 09/11/23.

The docs built without error with sphinx 7.2.6 locally and on readthedocs (currently pinned to 5.3). The only issue I have found is that the voted issues table (via datatables.net) fails to render, pressing F12 in the web browser shows the errors:

I am unable to fix this at present. We can either wait a bit longer to see if a future update fixes this or we could look to remove the voted tables use of datatables and just link directly to github (which now allows sorting by reactions).