Updated documentation

Description

This merge request adds a section for running the voxel grid downsampling and ray ground filter nodes.

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

Closes #84 (closed)