docs(realtimekit): add FAQ about demo app iframe usage

[ravindra-cloudflare]

Summary

Add FAQ about demo app iframe usage

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
/src/content/docs/realtime/	@cloudflare/pcx-technical-writing, @cloudflare/realtime, @cloudflare/RealtimeKit, @roerohan, @ravindra-cloudflare

[github-actions]

Preview URL: https://41cd0772.preview.developers.cloudflare.com
Preview Branch URL: https://realtimekit-demo-app-faq.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/realtime/realtimekit/faq/	https://realtimekit-demo-app-faq.preview.developers.cloudflare.com/realtime/realtimekit/faq/