feat(cdk/a11y): update docs for TreeKeyManager.

Author: BobobUnicorn

src/cdk/a11y/a11y.md

@@ -27,7 +27,7 @@ Navigation through options can be made to wrap via the `withWrap` method
 this.keyManager = new FocusKeyManager(...).withWrap();
 ```
 
-#### Types of key managers
+#### Types of list key managers
 
 There are two varieties of `ListKeyManager`, `FocusKeyManager` and `ActiveDescendantKeyManager`.
 
@@ -55,6 +55,64 @@ interface Highlightable extends ListKeyManagerOption {
 
 Each item must also have an ID bound to the listbox's or menu's `aria-activedescendant`.
 
+### TreeKeyManager
+
+`TreeKeyManager` manages the active option in a tree view. This is intended to be used with
+components that correspond to a `role="tree"` pattern.
+
+#### Basic usage
+
+Any component that uses a `TreeKeyManager` will generally do three things:
+* Create a `@ViewChildren` query for the tree items being managed.
+* Initialize the `TreeKeyManager`, passing in the options.
+* Forward keyboard events from the managed component to the `TreeKeyManager` via `onKeydown`.
+
+Each tree item should implement the `TreeKeyManagerItem` interface:
+```ts
+interface TreeKeyManagerItem {
+  /** Whether the item is disabled. */
+  isDisabled?: (() => boolean) | boolean;
+
+  /** The user-facing label for this item. */
+  getLabel?(): string;
+
+  /** Perform the main action (i.e. selection) for this item. */
+  activate(): void;
+
+  /** Retrieves the parent for this item. This is `null` if there is no parent. */
+  getParent(): TreeKeyManagerItem | null;
+
+  /** Retrieves the children for this item. */
+  getChildren(): TreeKeyManagerItem[] | Observable<TreeKeyManagerItem[]>;
+
+  /** Determines if the item is currently expanded. */
+  isExpanded: (() => boolean) | boolean;
+
+  /** Collapses the item, hiding its children. */
+  collapse(): void;
+
+  /** Expands the item, showing its children. */
+  expand(): void;
+
+  /**
+   * Focuses the item. This should provide some indication to the user that this item is focused.
+   */
+  focus(): void;
+}
+```
+
+#### Focus management
+
+The `TreeKeyManager` will handle focusing the appropriate item on keyboard interactions. However,
+the component should call `onInitialFocus` when the component is focused for the first time (i.e.
+when there is no active item).
+
+`tabindex` should also be set by the component when the active item changes. This can be listened to
+via the `change` property on the `TreeKeyManager`. In particular, the tree should only have a
+`tabindex` set if there is no active item, and should not have a `tabindex` set if there is an
+active item. Only the HTML node corresponding to the active item should have a `tabindex` set to
+`0`, with all other items set to `-1`.
+
 
 ### FocusTrap