docs: add early stage notice to POS SDK overview page

[devin-ai-integration]

docs: add early access notice to POS SDK overview page

Summary

Added a notice to the WalletConnect POS SDK overview page indicating that the service is in early access and requires contacting [email protected] to join the pilot program. The notice is placed after the overview section and before the Quickstart section using a <Note> component for visibility.

Review & Testing Checklist for Human

• Verify the <Note> component renders correctly and prominently on the preview deployment
• Confirm the messaging ("early access", "pilot program") aligns with company communication standards and product status
• Check that the placement (after overview, before quickstart) makes sense for user flow and doesn't disrupt the page structure
• Verify the mailto link to [email protected] works correctly

Test Plan

1. View the preview deployment for this PR (link in CI checks)
2. Navigate to Payments → Point of Sale (POS) SDK → Overview
3. Verify the notice appears prominently with correct styling
4. Click the [email protected] link to verify it opens an email client correctly

Notes

• Requested by: [email protected]
• Devin session: https://app.devin.ai/sessions/8ea0eeaf60dd4a7c91beee76eb0d6800
• The <Note> component is used for consistency with other callout boxes in the documentation
• Similar contact-sales messaging exists in wallet-sdk/chain-support/overview.mdx but uses a different format (bold text + link vs Note component)

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