docs: Update sphinx version to 5.0
ReadTheDocs has started complaining that the Sphinx version (previously 4.5.0) is not compatible. So let's upgrade to 5.0 to fix that.
Sphinx version error: The sphinxcontrib.applehelp extension used by this project needs at least Sphinx v5.0; it therefore cannot be built with this version.
Signed-off-by: Ryan Roberts ryan.roberts@arm.com