Check for Doxygen warnings in CI (BSP-429)

[igrr]

While looking at #266 I've noticed that although we do run Doxygen in CI, we don't actually fail the CI job when new warnings (e.g. function not documented) are introduced. Looking at the job log, there are already a lot of warnings, most of them about mismatch between the parameter names in the function declaration and in the Doxygen comment.

We should either

• change Doxygen configuration to fail on these warnings, e.g.

  WARN_AS_ERROR = YES
  

• or, check for the presence of "warning: " string in Doxygen logs, like esp-docs does this (with an optional ignore-list)