Document that ruff format is in Beta

[johnthagen]

The recent blog post mentions

Ruff’s Python formatter, available now in Beta

but the official docs, don't mention the beta fact:

• https://docs.astral.sh/ruff/formatter/

I think that it would be helpful to mention this in the docs as this is what most people will find and read rather than the original blog post, and it also gives a natural point for people to check to see when the formatter goes out of beta.

[charliermarsh]

I can add this, thanks!