docs: note standalone Nexus sample public preview

[litlep-nibbyt]

Do not merge before the SANO Public Preview window.

What was changed

Added an Availability section to the .NET SANO sample README, stating that SANO is in Public Preview and that Temporal aims to preserve backward compatibility and avoid breaking changes.

Why?

The .NET SANO sample did not previously explain the feature's release stage.

Checklist

1. How was this tested:

   • Reviewed the Markdown-only change.

2. Any docs updates needed?

   • No additional documentation changes in this repository.

Release version check: Before publishing, confirm that Temporal CLI v1.9.1 and Temporal Server v1.32.1 are still the latest stable releases that support SANO, then update any version references if needed.

Release verification note: SANO is controlled by the namespace-scoped server setting nexusoperation.enableStandalone, which defaults to false in Server 1.32.1. The sample PRs currently set it explicitly for local development. Before publishing, confirm whether the target stable CLI enables SANO by default in its dev server and whether that flag should remain in the sample instructions.

[CLAassistant]

All committers have signed the CLA.