Doxygen style is not verified in CI

[vyasr]

The pre-commit hook for doxygen is configured such that it exist without error if doxygen is not installed. This turns out to be the case in the checks dependency list that is used for the CI linting run. As a result, doxygen is not actually being run in CI. You can see the result in a recent CI run.

 doxygen-check............................................................Passed
- hook id: doxygen-check
- duration: 0s

warning: doxygen is not installed

doxygen should be installed in the environment. I tested this locally and I found that the current code has a number of doxygen errors that will need fixing since there's been some bit rot since the check was last run correctly and validated.