docs: added realtime kit documentation

[roerohan]

Summary

This MR updates the documentation for Cloudflare Realtime. This includes the following changes:

1. Moving the existing Realtime docs to "Realtime SFU"
2. Introducing RealtimeKit, and documenting it in the root folder. There are more users for RealtimeKit than there are for Realtime, so this documentation occupies the parent folder.
3. Creating the following pages for RealtimeKit documentation:
   3.1 Overview
   3.2 Introduction
   3.3 Concepts
   3.4 Getting Started
   3.5 Build your own UI

The plan is to move the RealtimeKit documentation from https://docs.realtime.cloudflare.com to this repository incrementally, so the changes in this MR might have links to https://docs.realtime.cloudflare.com. The goal is to move the introductory documentation for the RealtimeKit here in the first phase of changes.

Screenshots (optional)

This is what the root page for Realtime will look like once this PR is merged.

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• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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

[renandincer]

we're likely to have some broken links from moving the SFU contents to the SFU subdirectory, we might need to add some redirects

[roerohan]

we're likely to have some broken links from moving the SFU contents to the SFU subdirectory, we might need to add some redirects

I'll get these fixed in the next few commits.