Add Sybil examples to most classes in the root hoomd namespace.

[joaander]

Description

Add Sybil examples and revise documentation for all classes in the hoomd namespace (excluding hoomd.md and hoomd.hpmc which will be added in a future pull request).

Motivation and context

Many users prefer to copy and paste these minimal examples. Sybil tests the examples to ensure they operate.

How has this been tested?

I rendered the documentation locally. Pull request reviewers can click the "Details" link next to readthedocs.io in the checks below to see the new documentation.

Change log

Changed:

* Revise class documentation.
* Add more code snippets to the class documentation.

Checklist:

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

[b-butler]

This was/is a lot of work! Overall looks great and should be helpful to users.