Matrix keyboard module

[pjsg]

Fixes #3630

• [x] This PR is for the dev branch rather than for the release branch.
• [x] This PR is compliant with the other contributing guidelines as well (if not, please describe why).
• [x] I have thoroughly tested my contribution.
• [x] The code changes are reflected in the documentation at docs/*.

This adds the matrix module which supports simple matrix keypads. Simple interface -- it just calls a specified callback on key press (and/or key release).

[jmattsson]

Ah, one other review comment - the documentation needs to be linked in mkdocs.yml too :)