Add developer docs for cheatsheet

[pokey]

Basic docs introduced in #1718. In addition, we'd like to:

• [ ] How CI works
• [ ] Add note about the fact that the cheatsheet doesn't get rolled up when you run extension locally and how to fix that / run locally
• [ ] Switch cheatsheet json update to use cursorless repo root
• [ ] add note that you need to regen cheatsheet from development instance
• [ ] switch to using spaces instead of tabs
• [ ] document / look into the fact that for some reason you need to have root-level typescript watcher running for live reload to work effectively
• [ ] file issue to use default spoken forms for cheatsheet
  • [ ] need to have version of shapes / colors lists that are not restricted to what appears in vscode settings.json
  • [ ] need to have version of section heading lists that exclude the ones that are off by default
  • [ ] add test for cheatsheet that compares payload to defaults.json
  • [ ] mention to run spoken form test in cheatsheet developer docs
• [x] How to build locally
• [x] How to update the example json for local development
• [x] How to run locally