docs(realtimekit): add waiting room reference for mobile platforms

[Yash-Garg]

Summary

Added waiting room documentation for RealtimeKit Mobile SDKs.
Page Modified: http://developers.cloudflare.com/realtime/realtimekit/core/waiting-room/

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• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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.