Map all mkr1000 pins

[ojob]

Summary

Goal is to document all Arduino MKR1000 pins, and give hints on using them.

For instance, PWM configuration, with the TCC and Channel to use.

Checklist

• All new or modified code is well documented, especially public items
• No new warnings or clippy suggestions have been introduced - CI will deny clippy warnings by default! You may #[allow] certain lints where reasonable, but ideally justify those with a short comment.

If Adding a new Board

n/a

If Adding a new cargo feature to the HAL

no new feature

[ojob]

Following work on #878, here's a new branch to map all pins.

Still a question on PWM, that I have to explore some more: TCCx are trivial to use, TCx not so.

[ianrrees]

Still a question on PWM, that I have to explore some more: TCCx are trivial to use, TCx not so.

@ojob - I'm going to mark this as a draft for now, but let me know when the PWM questions are resolved and this is ready to go. Thanks!