add beta text in version 3 Payload CMS docs

[GerardPolloRebozado]

The docs for version 3 of payload used to show (beta) but this has changed.
I think it will be useful for people reading the docs to know that version 3 is a beta and its not stable

[tyteen4a03]

The docs for version 3 of payload used to show (beta) but this has changed. I think it will be useful for people reading the docs to know that version 3 is a beta and its not stable

Stable is dropping next week!