Docs: Add block toolbar component readme

[JustinyAhin]

Description

Added documentation for the block toolbar component (see #22891)

How has this been tested?

N/A, documentation only

Types of changes

Documentation

Checklist:

• [n/a] My code is tested.
• [n/a] My code follows the WordPress code style.
• [n/a] My code follows the accessibility standards.
• [n/a] My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[JustinyAhin]

@mkaz I'm wondering something.
For most of the uses cases of BlockToolbar, it is used with the hideDragHandle prop like this <BlockToolbar hideDragHandle />.

But I can't see nowhere what hideDragHandle does exactly. Any idea?

Thanks.

[mkaz]

It looks like it is used in the Block-Mover component here but hard to tell how.

@jasmussen can you add any additional context?

[jasmussen]

I can't speak with full authority on that, but I would imagine that is necessary for the situation where a block can't be moved. For example when there's only one block:

Other situations include where a block is locked otherwise.

[gziolo]

It looks like a good start 👍

We could also add a reference to BlockControls component (SlotFill) that typically serves as a way for blocks to inject their toolbar items inside this component.