Clean up messages documentation

Description

• The design document for the messages package was not properly rendered in the sidebar:

• The TODO notes were also being rendered, instead of being comments:

Notes for Reviewer

To check that the documentation is properly generated, type the following within ADE:

$ cd AutowareAuto
$ ./docs/.doxygen/build.py

and check it out via your favorite browser by going to docs/_build/html/index.html

Checklist

• Documentation was updated