doc: move the meson.build in as subdir()

Doing so means we can ditch the specific input list for doxygen and just copy all files over into our builddir/doc/ subdir, then use that subdirectory as input data.

This relies on meson putting a subdir() into a subdirectory in the build directory. This isn't technically guaranteed but I also suspect that if meson ever changes that, lots of other projects will break. Even in that case we should build just fine since we now filter for *.h and *.dox and don't copy any other doxygen-commented files into the builddir anyway.