[Docs]: Update block variations documentation about block scope

[ntsekouras]

Description

Based on this comment:

Sorry to add this to a closed issue. But I found it is very elusive in the documentation how to use the variations picker if block scope is chosen. I had to find it digging into the core src, comparing groups & columns.

For example, when creating variations on the core group block, the variations picker is not found. Yet, it allows me to choose block scope, while this doesnt do anything (because there is no <__experimentalBlockVariationPicker /> in core/group).

Would it be helpful if this is mentioned somewhere in the docs? (if you choose block scope, make sure there is a variation picker)

This PR tries to better clarify the usage of block scope in block variations. It also updates the BlockVariationPicker README file as is now linked from block variations docs page.

[gziolo]

That's a great enhancement to the documentation related to block variations. Thank you for bringing more clarity ❤️