Surface clear submission guidelines in advance

Patterns reviewers have complained that they're getting poor-quality submissions, but it's currently difficult to find expected quality standards until after one designs a pattern and clicks submit.

Current flow:

• They're invited to create a pattern
• On submit they're asked to check the guidelines
• Clicking that asks if they want to leave the site and lose their work :/

It would be kinder to set standards in advance, maybe a modal when they open the new pattern editor, or at least brief guides and a link to more info.

Here's one page with some vague rules. It's a start:
https://wordpress.org/patterns/

Or we could point to the patterns submission pathway:
https://make.wordpress.org/handbook/pathways/design/submit-block-patterns/

https://github.com/user-attachments/assets/d8ae453b-f517-4809-87cb-4d1c7bba9a78

Start by tracing the new pattern editor flow described in the issue, especially where users are invited to create a pattern and where submission guidelines are shown. Review the linked WordPress guidance for the expected standards. Done means contributors see clear guidance before designing or submitting and are not forced to leave the site and lose their work.