Add doxygen template to CMake docs target.

- Adds the template file to the target enabling easy
  edit via IDE editor.
- Drop a generated file comment at the beginning of
  the generated doxyfile in hopes of informing users it
  should not be modified.

BUG=aomedia:76,aomedia:559

Change-Id: Id4e2762a5dae145927e7b7f1f6689744150ba090