Develop guidelines for "what goes in the prelude"

Hmm. I really wish we had clear guidelines on "what goes in the prelude". My personal take is that this is rarely used (once per app generally), and should not be added to the prelude.

_Originally posted by @alice-i-cecile in https://github.com/bevyengine/bevy/issues/8778#issuecomment-1584823561_

Right now, contributors have difficulty deciding and agreeing on what items belong in the prelude. We should figure out a set of guidelines, then add it to CONTRIBUTING.md.

Some initial thoughts:

- extremely commonly used
- traits whose methods end users will call regularly (hi bevy_ecs extensions)
- names that are likely to clash with other common imports or user defined code are bad

Start by reading CONTRIBUTING.md and the prelude-related discussion linked from this issue. Turn the stated considerations into clear guidelines for contributors, including the initial thoughts about common use, regularly called traits, and name clashes. Done means the agreed guidance is added to CONTRIBUTING.md.

Written by the indexing model from the issue text.