Add help topics for keys and buttons

[blinry]

As per #1746, this is an implementation attempt.

The help for the keys is arranged in two columns to save some space:

Open questions:

• Should the topics be mentioned in the help texts for btn(p) and key(p)? I think probably not – for example, poke also doesn't mention the RAM and VRAM pages.
• Should the tables have headers, like RAM and VRAM?
• Should they be included when exporting the help?

[nesbox]

Should the topics be mentioned in the help texts for btn(p) and key(p)? I think probably not – for example, poke also doesn't mention the RAM and VRAM pages.
Should the tables have headers, like RAM and VRAM?

It's up to you :)

Should they be included when exporting the help?

I'd like to see it in the exported help :)

Thanks

[blinry]

I added them to the exported help, and I'm happy with this PR as-is! :)

[nesbox]

Thanks