Refactor Pro Micro pin mapping

[Nicell]

This PR refactors the Pro Micro pin mapping:

• Adds in the missing pins 18-21
• Renames pro_micro_d to pro_micro (pro_micro_d alias included for compatibility)
• Removes all uses of pro_micro_a in favor of pro_micro
• Updates documentation image + description of Pro Micro pins

[petejohanson]

Minor nitpick. Appreciate the backwards compatible approach, and getting all existing shields updated to help set proper examples moving forward.

[petejohanson]

Thoughts on how to prevent new shields getting merged in w/ the old names? Add an item to the check-list to make sure we're keeping an eye out for it?

[Nicell]

Thoughts on how to prevent new shields getting merged in w/ the old names? Add an item to the check-list to make sure we're keeping an eye out for it?

It doesn't look like there's a way to add a deprecation warning to our nexus nodes in DT, so no way to really detect usage. A check-list item seems like the best idea for now.

[petejohanson]

Thanks! This is much better.