Adding Easier Configuration for Dual-Side Encoders On Split Keyboards

[Vacant0mens]

As described and reviewed in this issue, the current way of configuring KMK to work with a split keyboard that has one or more rotary encoders on each side is to manually create the coord_mapping list. While not incredibly difficult, it can be tedious, depending on the configuration of the keyboard's hardware (among other things).

The changes in this PR update parts of the code in the Split class to allow for additional buttons (or in my case, encoder actions) to be automatically appended to the coord_mapping outside of the matrix defined by the two lists of Pin objects. Since the assignment of the key codes to those coordinate indexes is handled outside of the Split class itself, the coordinates can then be assigned to any compatible key-like hardware object via a Scanner class and subsequently assigned a keycode via the keyboard.keymap.

This provides a more accessible point of entry for adding accessories to split keyboards.

I've also added to the documentation around this topic and included examples, as well as updates to current descriptions to (hopefully) make this process easier for others going forward.

Feel free to give constructive feedback, ask any questions you may have, or submit any changes that may be needed. 👍

[xs5871]

Please also run make test and fix all the linting complaints.

[Vacant0mens]

BLK100 Black would make changes.
What does this error mean? 🤔
This is the line:
add_buttons=0 # add single-pin buttons, rotary encoder actions, etc. per-side.

[xs5871]

There's also a fix-formatting make command.
Line 44 is missing a comma at the end.

[xs5871]

I was wondering why the spellchecker didn't trip.

[Vacant0mens]

Hm. ... Should it have?