Document pattern for compare

[rebeccaalpert]

We wired up a comparison screen for Composer AI Studio - other tools may be interested in this pattern. We should document it to make it easy to reproduce.

Andy Braren would like to be able to point a developer to a link that helps them replicate what this demo can do.

For pattern examples, see https://www.patternfly.org/patterns/about-patterns/.

I'd want Erin and Kayla involved for the docs and design pieces, and maybe get a check from Eric on the a11y side to make sure it's not a total nightmare.