Add GMSO link and description to Related Tools in docs.

[chrisjonesBSU]

Description

Motivation and context

GMSO is starting to replace foyer as a user facing package. Also, the most up-to-date Hoomd writers now live in GMSO, and older versions are being removed from mBuild, so we should also link GMSO in the related tools section.

How has this been tested?

I built the docs without any errors. The new link is working.

Checklist:

• [x] I have reviewed the Contributor Guidelines.
• [x] I agree with the terms of the HOOMD-blue Contributor Agreement.
• [x] My name is on the list of contributors (sphinx-doc/credits.rst) in the pull request source branch.