Add withHistory page to docs, reorganize the History docs in general and update Summary.md

docs/libraries/slate-history/history.md

@@ -0,0 +1,27 @@
+# History
+
+The `History` object contains the undo and redo history for the editor.
+
+It can be accessed from an `Editor` instance as the property `history`.
+
+This property is only available on the `Editor` if the editor was instantiated using the `withHistory` method which adds undo/redo functionality to the Slate editor.
+
+```typescript
+export interface History {
+  redos: Batch[]
+  undos: Batch[]
+}
+
+interface Batch {
+  operations: Operation[]
+  selectionBefore: Range | null
+}
+```
+
+- [Static Methods](history.md#static-methods)
+
+## Static Methods
+
+#### `History.isHistory(value: any): value is History`
+
+Returns `true` if the passed in `value` is a `History` object and also acts as a type guard for `History`.