We should add a guide to writing shrinker-friendly strategies

[Zac-HD]

This would be one of our contributor guides, not the end-user documentation. While we're not perfect, in almost all cases applying these tricks on top of the public API would be a net negative... but they're sometimes essential underneath the public API.

Main points:

• Most users DO NOT need to know or use these tricks. If you think you might, here's how to tell.
  (i.e. you are implementing substantial custom logic with @composite or possibly data() and have had to manually shrink output reported by Hypothesis)
• The "shrink open" trick explained here and demonstrated here (from Increase variation in Unicode character generation #1621)
• An overview of valid-by-construction, data locality, and links to shrinking model. This can just be an edited version of comments on Custom strategies beyond composites #1381. I found the "generation choice sequences" framing from David's ICSE draft paper helpful in linking generation to shrinking, so we might also borrow from that.