fix: rename developer platform products for Realtime

[roerohan]

Summary

Previously, there used to be 2 products listed under Developer Platform in the product directory. They were Realtime and Realtime TURN Service.

This causes confusion as there are now 3 products under the Realtime umbrella. They are

1. RealtimeKit
2. Realtime SFU
3. Realtime TURN Service

Additionally, this PR also groups the Realtime services together (they were not grouped together before, screenshot below).

Screenshots (optional)

Before:

After:

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• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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

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