Add block inspector component readme (#24929)

* Add block inspector component readme

* Update block inspector component readme

packages/block-editor/src/components/block-inspector/README.md

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+# Block Inspector
+
+The Block inspector is one of the panels that is displayed in the editor; it is mainly used to view and modify the settings of the selected block.
+
+![Paragraph block inspector](https://make.wordpress.org/core/files/2020/08/paragraph-block-inspector.png)
+
+## Table of contents
+
+1. [Development guidelines](#development-guidelines)
+2. [Related components](#related-components)
+## Development guidelines
+
+### Usage
+
+Render the block inspector component.
+
+```jsx
+import { BlockInspector } from '@wordpress/components';
+
+const MyBlockInspector = () => <BlockInspector />
+```
+## Related components
+
+Block Editor components are components that can be used to compose the UI of your block editor. Thus, they can only be used under a [BlockEditorProvider](https://github.com/WordPress/gutenberg/blob/master/packages/block-editor/src/components/provider/README.md) in the components tree.