Add banner about o1labs docs site to o1js docs

[45930]

Every change ought to be adding

import MigrateBanner from '@site/src/components/snippets/migrate-banner.mdx';

<MigrateBanner />

to the top of a file, or defining the snippet:

import Admonition from '@theme/Admonition';

<Admonition type="info" title="New Home!">
  🚀 o1js reference documentation is now available at the new [o1Labs docs site](https://docs.o1labs.org/o1js).
</Admonition>

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Project	Deployment	Preview	Comments	Updated (UTC)
docs2	Ready	Preview	Comment	Sep 24, 2025 0:01am

[bleepbloopsify]

🎉