Since CMake 3.11, the supported method of integrating Doxygen into
CMake projects has been via the doxygen_add_docs() CMake function,
which (crucially) can be passed a target name to create and doesn't
rely on tons of hardcoding like our UseDoxygen.cmake and its
Doxyfile.in template did.
This means our docs (only) now require CMake 3.11 to generate, but it
allows Doxygen to be configured/used multiple times in the same CMake
configuration, removing a major barrier to superproject builds.
Also, made docs disableable via an ENABLE_AUDIO_DOCS option, and adopt
FeatureSummary into the build.
Remove Doxyfile.in template
CI: Don't autoinstall API docs
Delete Doxyfile.in
Docs: Use PROJECT_VERSION_FULL
Don't pre-process source files
