Update config.h boilerplate to use `#pragma once`

According to @fredizzimo, this is a safer and easier way to handle the
boilerplate.

Add Userspace mention

@drashna mentioned it'd be good to have a mention of the userspace in
the QMK structure section. Rather than rewrite the docs on userspace, I
chose to link to the existing documentation.

Reword the `config.h` section

This section didn't include the possibility of a user `config.h`, and it
wasn't clear exactly how the settings override works.

Update docs: `Makefile` -> `rules.mk`

I think this was a typo, or perhaps an older way to handle it.

Add I2C slave library

- Add I2C slave library for general use.

Added M10-B implementation

Added pin C6 to hardware PWM backlight

AJP10304 Atreus 50 layout (#3403)

UK78 Refactor: rename LAYOUT to LAYOUT_all (#3405)

UK78 refactor and Configurator support (#3395)

* Matrix update

* Keymap refactor

* Configurator support

* Readme cleanup

callum keymap updates (#3402)

* change window focus shortcuts

* hangul

* macos goodness

* re-arrange tab keys

* remove tab switching keys

* left hand func

* glorious macro pad

* workspace rotation keys

* tabbing hotkeys

mitosis:datagrok layout and readme improvements (#3400)

* mitosis:datagrok: improved tenkey layout; changelog + more in README

* mitosis:datagrok enable audio!

* mitosis:datagrok: underscore on right shift, rearrange some symbols

* mitosis:datagrok: add more descriptions to readme

* mitosis:datagrok: abuse space cadet to get equivalent of RSFT_T(KC_UNDS)

Add A Planck keymap with easier shifts and nav (#3399)

keymap BFO9000 built as 6x18 (#3398)

Configurator does not work with melody96 leds (#3396)

* move out led code from keymap.c to melody96.c

* remove the other led_set_user

Fix redox with DIODE_DIRECTION == ROW2COL (#3394)

The code for the redox keyboard was missing
an appropriate #ifdef in the matrix_init
function for the case where DIODE_DIRECTION == ROW2COL

Docs: add resources for git (#3390)

* add resources for git

* add more resources

Added E6v2 keymap & some minor changes (#3392)

* Changed HHKB ALT to GUI etc.

* Swapped DZ60 GUI and ALT key

* Added E6v2 keymap

* Fixed CAPS to LCTL on E6v2

Tada68: Updated links to external sites. (#3389)

Friend was having trouble with their tada since their build environment wasn’t setup.  Updated so they could access the links that were listed (old links 404’d)

Kmini refactor and Configurator support (#3388)

* default keymap refactor: QMK_KEYBOARD_H include; readability

* Configurator support

* info.json was missing a comma

* Added matrix functions to matrix.c per @drashna