Document how to add data driven configurations (#11502)

* describe how data driven configuration works

* Apply suggestions from code review

Co-authored-by: ridingqwerty <george.g.koenig@gmail.com>
Co-authored-by: Erovia <Erovia@users.noreply.github.com>

Co-authored-by: ridingqwerty <george.g.koenig@gmail.com>
Co-authored-by: Erovia <Erovia@users.noreply.github.com>

docs/_summary.md

@@ -159,6 +159,7 @@
     * [Contributing to QMK](contributing.md)
     * [Translating the QMK Docs](translating.md)
     * [Config Options](config_options.md)
+    * [Data Driven Configuration](data_driven_config.md)
     * [Make Documentation](getting_started_make_guide.md)
     * [Documentation Best Practices](documentation_best_practices.md)
     * [Documentation Templates](documentation_templates.md)

docs/data_driven_config.md

@@ -0,0 +1,59 @@
+# Data Driven Configuration
+
+This page describes how QMK's data driven JSON configuration system works. It is aimed at developers who want to work on QMK itself.
+
+## History
+
+Historically QMK has been configured through a combination of two mechanisms- `rules.mk` and `config.h`. While this worked well when QMK was only a handful of keyboards we've grown to encompass nearly 1500 supported keyboards. That extrapolates out to 6000 configuration files under `keyboards/` alone! The freeform nature of these files and the unique patterns people have used to avoid duplication have made ongoing maintenance a challenge, and a large number of our keyboards follow patterns that are outdated and sometimes harder to understand.
+
+We have also been working on bringing the power of QMK to people who aren't comformable with a CLI, and other projects such as VIA are working to make using QMK as easy as installing a program. These tools need information about how a keyboard is laid out or what pins and features are available so that users can take full advantage of QMK. We introduced `info.json` as a first step towards this. The QMK API is an effort to combine these 3 sources of information- `config.h`, `rules.mk`, and `info.json`- into a single source of truth that end-user tools can use.
+
+Now we have support for generating `rules.mk` and `config.h` values from `info.json`, allowing us to have a single source of truth. This will allow us to use automated tooling to maintain keyboards saving a lot of time and maintenance work.
+
+## Overview
+
+On the C side of things nothing really changes. When you need to create a new rule or define you follow the same process:
+
+1. Add it to `docs/config_options.md`
+1. Set a default in the appropriate core file
+1. Add your `ifdef` and/or `#ifdef` statements as needed
+
+You will then need to add support for your new configuration to `info.json`. The basic process is:
+
+1. Add it to the schema in `data/schemas/keyboards.jsonschema`
+1. Add code to extract it from `config.h`/`rules.mk` to `lib/python/qmk/info.py`
+1. Add code to generate it to one of:
+  * `lib/python/qmk/cli/generate/config_h.py`
+  * `lib/python/qmk/cli/generate/rules_mk.py`
+
+## Adding an option to info.json
+
+This section describes adding support for a `config.h`/`rules.mk` value to info.json.
+
+### Add it to the schema
+
+QMK maintains schema files in `data/schemas`. The values that go into keyboard-specific `info.json` files are kept in `keyboard.jsonschema`. Any value you want to make available to end users to edit must go in here.
+
+In some cases you can simply add a new top-level key. Some examples to follow are `keyboard_name`, `maintainer`, `processor`, and `url`. This is appropriate when your option is self-contained and not directly related to other options. In other cases you should group like options together in an `object`. This is particularly true when adding support for a feature. Some examples to follow for this are `indicators`, `matrix_pins`, and `rgblight`. If you are not sure how to integrate your new option(s) [open an issue](https://github.com/qmk/qmk_firmware/issues/new?assignees=&labels=cli%2C+python&template=other_issues.md&title=) or [join #cli on Discord](https://discord.gg/heQPAgy) and start a conversation there.
+
+### Add code to extract it
+
+Whenever QMK generates a complete `info.json` it extracts information from `config.h` and `rules.mk`. You will need to add code for your new config value to `lib/python/qmk/info.py`. Typically this means adding a new `_extract_<feature>()` function and then calling your function in either `_extract_config_h()` or `_extract_rules_mk()`.
+
+If you are not sure how to edit this file or are not comfortable with Python [open an issue](https://github.com/qmk/qmk_firmware/issues/new?assignees=&labels=cli%2C+python&template=other_issues.md&title=) or [join #cli on Discord](https://discord.gg/heQPAgy) and someone can help you with this part.
+
+### Add code to generate it
+
+The final piece of the puzzle is providing your new option to the build system. This is done by generating two files:
+
+* `.build/obj_<keyboard>/src/info_config.h`
+* `.build/obj_<keyboard>/src/rules.mk`
+
+These two files are generated by the code here:
+
+* `lib/python/qmk/cli/generate/config_h.py`
+* `lib/python/qmk/cli/generate/rules_mk.py`
+
+For `config.h` values you'll need to write a function for your rule(s) and call that function in `generate_config_h()`.
+
+If you have a new top-level `info.json` key for `rules.mk` you can simply add your keys to `info_to_rules` at the top of `lib/python/qmk/cli/generate/rules_mk.py`. Otherwise you'll need to create a new if block for your feature in `generate_rules_mk()`.