Remove References to Garnett in Package-wide Documentation

[tommy-waltmann]

Description

We have non-functional examples in our documentation demonstrating the use of garnett. This issue is to document that these examples need to be removed.

Motivation and Context

The garnett package is no longer actively being maintained. It doesn't support the latest major version of gsd, which is a very common file format used by freud's users to read/write trajectory data.