[RealtimeKit] Document store APIs for mobile

[harshs-dyte]

Summary

Adds mobile docs to the /realtime/realtimekit/broadcast-apis/ page

Depends on #27814 merged ✅

Screenshots (optional)

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• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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

[Yash-Garg]

Will wait for #27814 to merge.

[harshs-dyte]

Rebased on current production.