doc: add user facing section on swarms

[ianhi]

When investigating limiting rules on a state machine I figured out that what I wanted was swarm test. At first I didn't think hypothesis implemented this becuase a search for swarm only returns a single changelog entry: https://hypothesis.readthedocs.io/en/latest/search.html?q=swarm&check_keywords=yes&area=default

So this is a minimal version of the docs that I think would have gotten me to my current understanding quicker. Wasn't super confident on the jargon of test case vs run vs example so that last sentence might need some help to be less confusing.

[Liam-DeVoe]

It would be nice to document more of our internal exploration mechanisms. But any such documentation should live in an explanations page or possibly our not-yet-written internals guide, where it's clear it's philosophical and explanatory documentation, not part of the API contract.

[ianhi]

Sounds good. would you prefer it as it's own page, or as part of domains and distribution?