Add script to generate query fields documentation

- All lines matching "@QUERY_FIELDS_${resource}@" in the input will be
  replaced with a definition list describing the fields for $resource
- The core code is kept in the Sphinx extension, so that it could be
  used from there, too

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>