FindDoxygen: Fix support for project() and doxygen_add_doc() in a subdirectory

Internally, FindDoxygen.cmake uses ${PROJECT_BINARY_DIR} to look for the top-level CMakeDoxyfile.in file. This breaks when doxygen_add_doc() is invoked in a subdirectory that also calls project() beforehand. This modifies the PROJECT_BINARY_DIR variable to point to the respective subdirectory in the binary dir tree, which will not have the Doxygen files. Instead, it should use ${CMAKE_BINARY_DIR}.

Fixes: #17022 (closed)
Topic-rename: FindDoxygen-project-in-subdir