Explain how to navigate with the keyboard

### Problem

There was some feedback on social media that it would be nice if the system would explain how to navigate via keyboard when the screen reader is enabled

### Proposal

Include a page early on about how to navigate the shell with keyboard shortcuts. The most important ones seem to be:

- [ ] accessing the applications menu
- [ ] Switching between open apps
- [ ] Navigating panel indicators

It might be worth always showing this whether or not screen reader is detected. There might be other kinds of motor disabilities that would make navigating by keyboard easier than mouse/touchpad and there are just some folks that prefer to navigate via keyboard

### Prior Art (Optional)

I think in Ubuntu (not sure if it’s in GNOME upstream) they do something with a11y during onboarding (or maybe it was installer?) that might be worth looking at