add workflow to test doxygen build

[stephprince]

See discussion and commits in related PR here: #59

Add separate action to test docs build on PRs (without deploying the docs and with doxygen warnings elevated to errors) See the discussion here #56 (comment). The Doxygen settings to elevate warnings are here https://www.doxygen.nl/manual/config.html#cfg_warn_as_error. I think we want to set WARN_AS_ERROR = FAIL_ON_WARNINGS_PRINT so that Doxygen runs to the end, prints warnings as a single block, but then exits with a an error code. In this way we can see all the warnings (instead of failing after the first one) and all warnings are listed together to make them easier to review.

[stephprince]

there are currently warnings likely related to the doxygen version - this should also be updated