feat(realtimekit): add custom control bar docs for mobile platforms

[thisisamank]

Summary

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• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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

[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/product-owners, @cloudflare/realtime, @cloudflare/RealtimeKit, @roerohan, @ravindra-cloudflare

[github-actions]

CI run failed: build logs

[github-actions]

Broken Links

Found 1 broken link(s) across 1 file(s).

File	Link	Position	Error
realtime/realtimekit/ui-kit/custom-controlbar.mdx	/realtime/realtimekit/ui-kit/api-reference/flutter/rtk-provider/	833:40	invalid link

[github-actions]

Preview URL: https://274cb59e.preview.developers.cloudflare.com
Preview Branch URL: https://ak-custom-control-bar.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/realtime/realtimekit/ui-kit/custom-controlbar/	https://ak-custom-control-bar.preview.developers.cloudflare.com/realtime/realtimekit/ui-kit/custom-controlbar/

[harshs-dyte]

LGTM

[harshs-dyte]

Needs to be rebased on #29691 to resolve the CI failure about a dead link (RtkProvider doc doesn't exist in the production branch yet)