Added sitemap to robots.txt

[Darylgolden]

Overview: What does this pull request change?

I noticed that the sitemap information wasn't added automatically to robots.txt for the documentation now that we're defining it ourselves. This PR adds it, just in case.

Motivation and Explanation: Why and how do your changes improve the library?

Links to added or changed documentation pages

Further Information and Comments

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested