Further refactoring of joystick feature

[fauxpark]

Description

Moving almost everything out of process_keycode/; process_joystick.c now solely handles the joystick buttons, though now it's able to be later expanded to add keycodes for axes too, as discussed in #13344. To this end I've chopped up process_joystick_analogread_quantum() into:

int16_t joystick_read_axis(uint8_t axis);
void joystick_read_axes(void);
void joystick_set_axis(uint8_t axis, int16_t value);

Docs have been updated to demonstrate use -- the existing example doesn't appear to actually do anything.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[fauxpark]

All the ARM onekeys are failing on the joystick keymap, since they don't have ADC stuff setup. Everything else appears to be working, though.