[traits] rename CoordTrait::nth_unchecked -> nth_or_panic to match conventional semantics

[michaelkirk]

• [x] I agree to follow the project's code of conduct.

• [x] I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

unchecked implies it's an unsafe method, like Vec::get_unsafe. It's how we use the term elsewhere in this api, e.g. see this discussion https://github.com/georust/geojson/pull/245#discussion_r1821255299

I also cleaned up some stale docs.

I opted not to add foo_or_panic methods to the various collections in this PR, as it seemed like a slightly different issue that could be followed up separately.

[michaelkirk]

I've restored nth_unchecked, marked it unsafe and by default it just calls nth_or_panic.

Please take another look.