Require all source code files to be included in Doxygen documentation

[menon-karthik]

We currently have the Doxygen Github actions set up so that undocumented code makes the action fail and is automatically blocked from being merged. However, we recently discovered that undocumented code can sneak in if it is in a source file which does not have a @file tag at the top and Doxygen doesn't see it as a result. It would be great to block this loop-hole and automatically enforce that every source file is included in the Doxygen build.