Describe the issue linked to the documentation

The link in the warning about sphinx versions at the end of 'Building the documentation' is to a Github search that uses a now unrecognised qualifier. Also I think there has been a change in how dependencies in CI are dealt with now (original PR, #10685, adding this section is 6 years old) and even when the qualifier syntax is updated, that search no longer shows the circle CI sphinx version.

Suggest a potential alternative/fix

Could this link just be to the file build_tools/circle/doc_environment.yml ? Or link to github search for 'sphinx' within this file?