Add Prefect 3 announcement bar to 2.x docs

[discdiver]

Screenshot:

Closes #13960

Checklist

• [X] This pull request includes a label categorizing the change e.g. maintenance, fix, feature, enhancement, docs.
• [X] This pull request references any related issue by including "closes <link to issue>"
  • If no issue exists and your change is not a small fix, please create an issue first.
• [ ] If this pull request adds new functionality, it includes unit tests that cover the changes
• [ ] If this pull request removes docs files, it includes redirect settings in mint.json.
• [ ] If this pull request adds functions or classes, it includes helpful docstrings.