Add entity-provider functions in the core-data documentation

[aurooba]

What?

This PR updates the documentation for the core-data package to include the hooks and providers in the core-data/src/entity-provider.js file.

Why?

Folks outside of just core can use functions like useEntityProp and they should be documented and available in the handbook

How?

So I had to make a small code change in order for this to work well. the EntityProvider function was being exported as a default function. As far as I could tell, there's no way for me to provide an alternative name so docgen would name this function default in the markdown file, which is no good.

So I switched it so that it's no longer a default export and just a normal named export from the file. From what I can tell, this doesn't cause any issues anywhere, but I'd love for someone to confirm this!

[bacoords]

+1 to getting this reviewed and merged!