Docs: Add Block Lock Readme

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

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+# Block Lock
+
+The `BlockLock` components render the UI for locking a block's movement, editing and removal, and the `useBlockLock` hook exposes a block's current lock state. Movement and removal can be locked on any block; the editing option is only offered for an allowlist of block types, currently `core/navigation`.
+
+These are private components of the `@wordpress/block-editor` package. They are not exported publicly and are meant to be consumed from within the package only.
+
+## Development guidelines
+
+### Usage
+
+Renders a toolbar button that opens the block lock modal.
+
+```jsx
+/**
+ * Internal dependencies
+ */
+import { BlockLockToolbar } from '../block-lock';
+
+const MyBlockToolbar = ( { clientId } ) => (
+	<BlockLockToolbar clientId={ clientId } />
+);
+```
+
+### Components
+
+#### `BlockLockMenuItem`
+
+A menu item that opens the block lock modal. Renders nothing when the block type does not allow locking.
+
+#### `BlockLockModal`
+
+A modal that lets the user choose which lock features to apply to the block.
+
+#### `BlockLockToolbar`
+
+A toolbar button that opens the block lock modal. Renders nothing until the block is locked.
+
+### Props
+
+#### `clientId`
+
+-   **Type:** `String`
+-   **Required:** Yes
+
+The client ID of the block. Accepted by all three components.
+
+#### `onClose`
+
+-   **Type:** `Function`
+-   **Required:** Yes
+
+A callback invoked when the modal is dismissed. Accepted by `BlockLockModal` only.
+
+### `useBlockLock`
+
+A hook that returns the lock state of a block. It accepts the block's client ID and returns an object with the following properties.
+
+-   `isEditLocked` (`Boolean`): Whether editing the block is locked.
+-   `isMoveLocked` (`Boolean`): Whether moving the block is locked.
+-   `isRemoveLocked` (`Boolean`): Whether removing the block is locked.
+-   `canLock` (`Boolean`): Whether the block type allows locking.
+-   `isLocked` (`Boolean`): Whether any of the block's lock features are applied.
+
+## 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/HEAD/packages/block-editor/src/components/provider/README.md) in the components tree.