Add support for the latest Sphinx release v5.1

[markkuriekkinen]

A-plus-rst-tools has been tested and used on Sphinx v4.1 and a bit on v4.3. Newer Sphinx versions have been released. Are there any compatibility issues? Check it out, test, and document. New versions of the apluslms/compile-rst container should be published with the new Sphinx version installed.

https://github.com/apluslms/compile-rst

• https://www.sphinx-doc.org/en/master/changes.html
• https://github.com/sphinx-doc/sphinx/releases