docs: note standalone Nexus operations public preview

[litlep-nibbyt]

What was changed

Added a short Standalone Nexus Operations entry to the Java SDK README's existing documentation list. It links to the Java guide and states that SANO is in Public Preview, with an aim to preserve backward compatibility and avoid breaking changes.

Why?

The README should document SANO's release stage without elevating it to a top-level SDK feature.

Do not merge before the SANO Public Preview window.

Checklist

1. How was this tested:

   • Reviewed the rendered Markdown and confirmed the documentation link resolves.

2. Any docs updates needed?

   • Public Temporal documentation is being updated separately.

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

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[Quinn-With-Two-Ns]

We generally don't like specific features or their status here , we link to the docs above at https://docs.temporal.io/docs/java/introduction and that covers specific features and their release status

[litlep-nibbyt]

Closing because the Java README will not include a standalone SANO availability note.