Generated the plugin API headers using a shell script.
Fixed some doxygen comment mistakes in the headers.
Added a cmake conveninence target to generate the docs into $BUILD_DIR/docs
Added a main page for the API docs.
Included all of the existing group .md files into the CMake target
Leveraged moxygen 2.1.11's fixes to produce the full API docs in a single go
Removed the list of output .md files from the CMake target and switched to a
stamp file to avoid unnecessary rebuilds of the docs when the list of .md
files changes.
Addressed various review comments.
