Fix Doxygen warnings for undocumented C++ code

[calcmogul]

We recently went through the effort to fix (or suppress in the case of the HAL) all of JavaDoc 17's warnings for undocumented Java code (https://github.com/wpilibsuite/allwpilib/pull/6220). We should do the same for C++.

The following flags in the doxygen block of docs/build.gradle should be set to true:

    warnings false
    warn_if_undocumented false

[spacey-sooty]

Should we resolve warnings in third party utils? A large amount of the warnings appear to be from third party libraries

[calcmogul]

We should suppress those.