Logfire annoucement (#9362)

Author: samuelcolvin

docs/theme/announce.html

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-<a href="https://docs.pydantic.dev/2.0/blog/pydantic-v2-final/">Pydantic V2</a> is here 🚀! Upgrading an existing app? See the <a href="https://docs.pydantic.dev/2.0/migration/">Migration Guide</a> for tips on essential changes from Pydantic V1!
+<b>We're live!</b> <a href="https://pydantic.dev/logfire">Pydantic Logfire</a> is out in open beta! 🎉<br>
+Logfire is a new observability tool for Python, from the creators of Pydantic, with great Pydantic support.
+Please try it, and tell us <a href="https://docs.pydantic.dev/logfire/help/">what you think</a>!