Make it easier for users to see their alphabet

[pokey]

In addition to https://github.com/cursorless-dev/cursorless/issues/936, we should have an easy way for the user to reference their own custom alphabet. This could take the form of

• A cheatsheet section, and/or
• A Cursorless sidebar panel
• Something else?

I would lean slightly towards a sidebar panel, because then it's easy to keep it right there, and we can render it nicely, but I don't feel strongly