Describe our attrs usage in contributor guide

[jakkdl]

#3114 made me think that we should have a short blurb in the contributors guide on our attrs usage. I remember it being very confusing when I started contributing, and #3114 adds another pitfall (though thankfully with a test), and tbh I'm still not fully sure myself when to use what.

Possible content:

1. why we use attrs in the first place(?)
2. that we prefer underscored attribute names, with an alias (and why pyright makes the alias needed)
3. when to use slots=False
4. when to use eq=False
5. when to use @frozen

The latter three are fairly common, but not universal, and I have a suspicion that we may not be fully consistent. Addressing the points could include pointing to official attrs docs, issues/PRs discussing pros/cons, or summarizing info that's happened in a lot of different places.

[Zac-HD]

I just noticed that we've dropped the aliases from CancelScope, exactly because pyright started complaining when I updated Trio 😓

Maybe we should have a tiny flake8 plugin or grep command in CI to check for that? And we should also report it upstream to pyright, the best fix would be there.

[jakkdl]

I just noticed that we've dropped the aliases from CancelScope, exactly because pyright started complaining when I updated Trio 😓

Maybe we should have a tiny flake8 plugin or grep command in CI to check for that? And we should also report it upstream to pyright, the best fix would be there.

yeah that's #3114