docs(asyncapi): update retry extension docs to use x-fern-retries and mark as unsupported

[devin-ai-integration]

docs(asyncapi): update retry docs to use x-fern-retries and mark unsupported

Summary

Updates the AsyncAPI retry extension documentation to align with issue #10597 option #1:

• Changes extension name from x-fern-retry (singular) to x-fern-retries (plural) to match OpenAPI naming convention
• Adds prominent warning that this extension is not supported for AsyncAPI WebSocket channels
• Explains why retry semantics don't apply to persistent WebSocket connections (unlike HTTP request-response)
• Updates example to show the extension will be ignored with a warning during import

This documentation change corresponds to the CLI implementation in fern-api/fern#10598, which adds a warning when x-fern-retries is detected in AsyncAPI specs.

Review & Testing Checklist for Human

• Verify warning message accuracy: Check that the warning message described in the docs matches what the CLI actually emits (see fix(cli): warn on AsyncAPI x-fern-retries extension fern#10598)
• Test the OpenAPI retries link: Verify that /api-definition/openapi/extensions/retries resolves correctly in the docs site
• Confirm naming convention: Verify that x-fern-retries (plural) is indeed the correct naming used in OpenAPI extension docs
• Review technical explanation: Confirm that the explanation about WebSocket vs HTTP retry semantics is accurate and clear

Notes

• This PR is part of implementing option feat(ci): Add stale-bot #1 from issue AsyncAPI retry extension inconsistency: documented but not implemented fern#10597
• The corresponding CLI implementation is in fix(cli): warn on AsyncAPI x-fern-retries extension fern#10598 (already merged and passing CI)
• Session: https://app.devin.ai/sessions/1f3f25ab839e41ec8e1b9e0ead287b12
• Requested by: danny@buildwithfern.com (@dannysheridan)

[devin-ai-integration]

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[github-actions]

🚫 [vale] _{reported by reviewdog 🐶}
[Microsoft.Contractions] Use 'isn't' instead of 'is not'.

[github-actions]

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