Add an explainer for the UndoManager API.

[whsieh]

• UndoManager/README.md: Added.

Add the explainer.

• UndoManager/demo/drawing.js: Added.
• UndoManager/demo/editing.js: Added.
• UndoManager/demo/index.css: Added.
• UndoManager/demo/index.html: Added.
• UndoManager/demo/index.js: Added.

Add a simple demo page that uses the experimental UndoManager API. This demo page implements a simple drawing web application, where each stylus stroke is undo-able and redo-able in a way that integrates with system undo/redo affordances as well (e.g. iPad undo/redo arrow buttons).

• UndoManager/edit-menu-bar-macOS.png: Added.
• UndoManager/shake-to-undo-iOS.png: Added.
• UndoManager/undo-system-ui-iOS.png: Added.

Add example screenshots (referenced in the explainer's markdown).

[whsieh]

Please also add a link to the new explainer to the README.md in the root directory. Thanks!

Thanks for taking a look! I added a link to UndoManager/ in the updated PR. I also added a quick recording of the demo page as well in this updated version.