docs: keyboard tutorial

[mcdurdin]

@DavidLRowe has taken up working on the draft keyboard tutorial rewrite.

This is a list of related issues that may help in that project:

• Suggestion: Clarify that .KMP files are used in KAB and how to update/create them #1175
• chore: add keyboard developer documentation on keying examples for .keyboard_info #356
• Link to keyboards repository for examples #148