Cleanup docs warnings

[sf1919]

Describe the outcome that is desired.
We are getting a lot of warnings when building the documentation. The ideal would be to have 0 warnings

Describe any solutions you are considering
Clean up documentation .

[peterfpeterson]

My preference is that part of properly collecting the release notes together is to turn on -W on the sphinx build which will convert warnings to errors. It'd be even better if we could fix up the amalgamate directive (written by us) to fix up the bullet lists when it collects them so we could have the flag turned on all the time. I found another project that simplified turning on the functionality as a cmake option. Somewhere in the top leve add

if (SPHINX_WARNINGS_AS_ERRORS)
  set(SPHINX_WARNINGS_AS_ERRORS_FLAG "-W")
else()
  set(SPHINX_WARNINGS_AS_ERRORS_FLAG "")
endif()

then when the sphinx targets get added, do something like add ${SPHINX_WARNINGS_AS_ERRORS_FLAG} to the list of arguments.

[mantid-builder]

Fixed by #35602