feat: document ref for self-referencing

[czabaj]

I encountered a problem with valid JS code that was hard to translate into rescript - the problem is discussed here in the forum - it turned out the ref can help with self-referencing.

I promised to add this trick to the documentation, so here it is. Feel free to criticize 😸

I currently added it just to the v11 docs, once this passes validation, I will copy that into other versions as well. I assume it is useful for all versions where mutable let assignment exists?

Here is how it renders

[vercel]

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