docs: Document how to access current url from within root.tsx

[srapport]

Document how to access current url from within root.tsx Thanks to @shairez.

What is it?

• [ ] Feature / enhancement
• [ ] Bug
• [X] Docs / tests / types / typos

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🔨 Latest commit	8ca98d5380afb85f514cd60b50d7388112b2da11

[shairez]

thanks @srapport ! 🙏