Fix Doxygen warnings for C++ code

[spacey-sooty]

Resolves #6221

[calcmogul]

I think Doxygen is failing to parse requires clauses. Here's how I worked around them in Sleipnir:

// FIXME: Doxygen gives internal inconsistency errors:
//   scope for class sleipnir::fmt::formatter< Derived, CharT > not found!
//   scope for class sleipnir::fmt::formatter< sleipnir::Variable > not found!
//   scope for class sleipnir::fmt::formatter< T > not found!

//! @cond Doxygen_Suppress

Badly parsed code here

//! @endcond

[sciencewhiz]

It looks like the latest Doxygen (1.10.0) has a fix for parsing requires

[spacey-sooty]

If so that would probably be a good reason to upgrade. Might also fix the parsing bugs for other parts. I'll have a go fixing the warning around WPIWarnings.mac and WPIErrors.mac without a supression and wait for the WPILib mirror of Doxygen releases to have latest Doxygen before trying to fix everything else.