Make keyboard shortcuts a bit more discoverable

[domenic]

There is some cool functionality in the keyboard shortcuts! But you can only learn about them by knowing to press ?.

I suggest adding some kind of well-labeled button that opens the keyboard shortcuts popup, e.g. in the sidebar somewhere. (Perhaps floating at the bottom, separate from the TOC.)

Another approach would be to add buttons for all the various functionality (e.g. a toggle multipage button, a show user code annotations button, and a show keyboard shortcuts button). That would be more of a design challenge but would have the added bonus of making the user-code annotation functionality available on mobile.