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Improve support and docs for ADC driver (#7191)

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* Improve support and docs for ADC driver

* Comment ADC channels

* Move to Makers and Modders section, and fix usage instructions

* Flesh out intro

* Superscript 328P note

* Fix pin_to_mux LUT

* Support USB64/1287 as well

* analogReadPin() defaults to 0V mux on invalid pin

* Update pinToMux() function documentation

* Dot

* Accept (some of) the `qmk cformat` changes

* Do clang-format properly

* More wording tweaks

* Link to encoder docs
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* [Hand Wiring Guide](hand_wire.md)
* [ISP Flashing Guide](isp_flashing_guide.md)
* [ARM Debugging Guide](arm_debugging.md)
* [ADC Driver](adc_driver.md)
* [I2C Driver](i2c_driver.md)
* [WS2812 Driver](ws2812_driver.md)
* [GPIO Controls](internals_gpio_control.md)

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# ADC Driver
QMK can leverage the Analog-to-Digital Converter (ADC) on supported MCUs to measure voltages on certain pins. This can be useful for implementing things such as battery level indicators for Bluetooth keyboards, or volume controls using a potentiometer, as opposed to a [rotary encoder](feature_encoders.md).
This driver is currently AVR-only. The values returned are 10-bit integers (0-1023) mapped between 0V and VCC (usually 5V or 3.3V).
## Usage
To use this driver, add the following to your `rules.mk`:
```make
SRC += analog.c
```
Then place this include at the top of your code:
```c
#include "analog.h"
```
## Channels
|Channel|AT90USB64/128|ATmega16/32U4|ATmega32A|ATmega328P|
|-------|-------------|-------------|---------|----------|
|0 |`F0` |`F0` |`A0` |`C0` |
|1 |`F1` |`F1` |`A1` |`C1` |
|2 |`F2` | |`A2` |`C2` |
|3 |`F3` | |`A3` |`C3` |
|4 |`F4` |`F4` |`A4` |`C4` |
|5 |`F5` |`F5` |`A5` |`C5` |
|6 |`F6` |`F6` |`A6` |* |
|7 |`F7` |`F7` |`A7` |* |
|8 | |`D4` | | |
|9 | |`D6` | | |
|10 | |`D7` | | |
|11 | |`B4` | | |
|12 | |`B5` | | |
|13 | |`B6` | | |
<sup>\* The ATmega328P possesses two extra ADC channels; however, they are not present on the DIP pinout, and are not shared with GPIO pins. You can use `adc_read()` directly to gain access to these.</sup>
## Functions
|Function |Description |
|----------------------------|-------------------------------------------------------------------------------------------------------------------|
|`analogReference(mode)` |Sets the analog voltage reference source. Must be one of `ADC_REF_EXTERNAL`, `ADC_REF_POWER` or `ADC_REF_INTERNAL`.|
|`analogRead(pin)` |Reads the value from the specified Arduino pin, eg. `4` for ADC6 on the ATmega32U4. |
|`analogReadPin(pin)` |Reads the value from the specified QMK pin, eg. `F6` for ADC6 on the ATmega32U4. |
|`pinToMux(pin)` |Translates a given QMK pin to a mux value. If an unsupported pin is given, returns the mux value for "0V (GND)". |
|`adc_read(mux)` |Reads the value from the ADC according to the specified mux. See your MCU's datasheet for more information. |