realtime: Add agents page

[git-bruh]

Summary

This adds a getting started page for the Realtime Agents SDK

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  - [ ] If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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