Define an official grammar for documentation and site in Bevy

## How can Bevy's documentation be improved?

While reviewing some PRs from Migration Guide and Bevy site in general I saw a lot of fixes which where just converting from "en-GB" to "en-UK". Since Bevy doesn't have stated somewhere (AFAIK) which one is the "official" Bevy one, those can't be considered valid fixes, since both are valid english.

It would be nice if Bevy explicit define somewhere, maybe on `CONTRIBUTING.md`, which gamar to use while writing docs or any formal communication or even if it allows any "en" variation, and such, we don't consider those conversions to be valid fixes anymore.

Start by reviewing CONTRIBUTING.md and the existing documentation and site guidance to determine where a language policy belongs. Resolve whether Bevy should adopt en-GB, en-UK, or permit multiple English variants, then document the agreed rule and explain which wording changes are valid; done means contributors have a clear official standard to follow.

Written by the indexing model from the issue text.