Trim down README.
README: Add instructions on how to build documentation
Add requirements/docs.txt for just documentation dependencies
Add link to RTD at top of README.
Remove unncessary comma
Merge branch 'bug/15195' into develop

* bug/15195:
  Remove unncessary comma
  Add link to RTD at top of README.
  Add requirements/docs.txt for just documentation dependencies
  README: Add instructions on how to build documentation
  Trim down README.