Add/inserter list item component readme

[JustinyAhin]

Description

Added documentation for the inserter list item 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]

@ntsekouras maybe you can help me on this ... I'm having a hard time to identify what are the required props for this component.

[ntsekouras]

Hey @JustinyAhin! The InserterListItem component it's actually a single element that shows a block in quick appender and the main appender list. It's only used for now in these places with a single entry point here:

gutenberg/packages/block-editor/src/components/block-types-list/index.js

Line 52 in 9ced21b

<InserterListItem

You can see from the code above, the usage of many needed props in GB core.

I'm not really sure if the composite is required and if is tightly coupled with Reakit parent there.. (import { Composite, useCompositeState } from 'reakit';

(https://reakit.io/docs/composite/). @youknowriad any input here, since I saw you made that addition for better keyboard navigation.