docs: Add docs on hardware metadata files.

docs/docs/development/hardware-metadata-files.md

@@ -0,0 +1,111 @@
+---
+title: Hardware Metadata Files
+---
+
+## Overview
+
+ZMK leverages an additional metadata YAML file for all boards and shields to provide high level information about the hardware to be leveraged by setup scripts/utilities, proper inclusion in the website hardware list, etc.
+
+The naming convention for metadata files is `{item_id}.zmk.yml`, where the `item_id` is the board/shield identifier, including version information but excluding any optional split `_left`/`_right` suffix, e.g. `corne.zmk.yml` or `nrfmicro_11.zmk.yml`.
+
+## Example File
+
+Here is a sample `corne.zmk.yml` file from the repository:
+
+```yaml
+file_format: "1"
+id: corne
+name: Corne
+type: shield
+url: https://github.com/foostan/crkbd/
+requires:
+  - pro_micro
+exposes:
+  - i2c_oled
+features:
+  - keys
+  - display
+siblings:
+  - corne_left
+  - corne_right
+```
+
+## Schema
+
+ZMK leverages a [JSON Schema](https://github.com/zmkfirmware/zmk/blob/main/schema/hardware-metadata.schema.json) file to validate metadata files and sure all required properties are present, and all other optional properties provided conform to the expected format. You can validate all metadata files in the repository by running `west metadata check` from your configured ZMK repository.
+
+## File Format
+
+The first line of every metadata file should contain the file format. As of today, the only acceptable file format is `1`, which should be written like:
+
+```yaml
+file_format: "1"
+```
+
+:::note
+
+ZMK plans to expand on the initial based set of metadata properties, while maintaining backwards compatibility. In particular, new _optional_ properties may be added to file format `1` as time progress, and when new set of properties is settled upon as being required moving forward, a new major version of the file format will be created to encompass those changes.
+
+:::
+
+### Item ID and Name
+
+All metadata files should then contain two key pieces of identifying information, the `id` for the item, and the `name` which used a the human readable display name in various UI locations:
+
+```yaml
+id: corne
+name: Corne
+```
+
+### Item Types
+
+Each metadata file includes a `type` property uniquely identifying the type of item, be it `board`, `shield`, or the less frequently needed `interconnect` (which is used to document generic hardware interconnects like the Pro Micro format):
+
+```yaml
+type: shield
+```
+
+### URL
+
+The `url` property should contain the canonical URL used to learn more about the board/shield, e.g. the main vendor website or GitHub repository for a given keyboard/controller:
+
+```yaml
+url: https://github.com/foostan/crkbd/
+```
+
+### Interconnect Requires/Exposes
+
+For boards and shields, one of the key pieces of high level information is compatibility between the two items. In particular, a board usually exposes one ore more "interconnects", the physical location/type of connections available, and their assigned possible uses (e.g. GPIO, power, ground, i2c, etc). Similarly, a shield is usually designed around one (or sometimes more) "interconnects" that allow it to connect to one of those boards.
+
+In ZMK, we encode both of those scenarios with the `exposes` and `requires` properties, respectively. For example, for a Corne shield that requires a Pro Micro compatible controller to function, and simultaneously exposes a four pin header to be used by standard i2c OLED modules, the metadata file contains:
+
+```yaml
+requires:
+  - pro_micro
+exposes:
+  - i2c_oled
+```
+
+### Features
+
+Boards and shields should document the sets of hardware features found on them using the `features` array. There is a fixed enum of possible values to use here, which will be expanded over time. The current set of possible `features` values is:
+
+- `keys` - Any board or shield that contains keyboard keys should include this feature. It is a central feature used to determine if we have a "complete combination" for ZMK to produce a keyboard firmware when performig setup.
+- `display` - Indicates the hardware includes a display for use with the ZMK display functionality.
+- `encoder` - Indicates the hardware contains one or more (possibly optional) rotary encoders.
+- `underglow` - Indicates the hardware includes underglow LEDs.
+- `pointer` (future) - Used to indicate the hardware includes one or more pointer inputs, e.g. joystick, touchpad, or trackpoint.
+
+### Siblings
+
+The `siblings` array is used to identify multiple hardware items designed to be used together as one logical device. Right now, that primarily is used to identify the two halves of a split keyboard, but future enhancements will include more complicated and flexible combinations.
+
+The array should contrain the complete harware IDs of the siblings that combine in the logical device, e.g.:
+
+```yaml
+siblings:
+  - corne_left
+  - corne_right
+```
+
+Future versions of the metadata file format will be expanded to allow documenting any specifics of each sibling that are unique, e.g. if only the left side contains the `encoder` feature.

docs/docs/development/new-shield.md

@@ -368,6 +368,32 @@ Further documentation on behaviors and bindings is forthcoming, but a summary of
 - `trans` is the "transparent" behavior, useful to be place in higher layers above `mo` bindings to be sure the key release is handled by the lower layer. No binding arguments are required.
 - `mt` is the "mod-tap" behavior, and takes two binding arguments, the modifier to use if held, and the keycode to send if tapped.
 
+## Metadata
+
+ZMK leverages an additional metadata YAML file for all boards and shields to provide high level information about the hardware to be leveraged by setup scripts/utilities, proper inclusion in the website hardware list, etc.
+
+The naming convention for metadata files is `{item_id}.zmk.yml`, where the `item_id` is the board/shield identifier, including version information but excluding any optional split `_left`/`_right` suffix, e.g. `corne.zmk.yml` or `nrfmicro_11.zmk.yml`.
+
+Here is a sample `corne.zmk.yml` file from the repository:
+
+```yaml
+file_format: "1"
+id: corne
+name: Corne
+type: shield
+url: https://github.com/foostan/crkbd/
+requires: [pro_micro]
+exposes: [i2c_oled]
+features:
+  - keys
+  - display
+siblings:
+  - corne_left
+  - corne_right
+```
+
+You should place a properly named `foo.zmk.yml` file in the directory next to your other shield values, and fill it out completely and accurately. See [Hardware Metadata Files](/docs/development/hardware-metadata-files) for the full details.
+
 ## Adding Features
 
 ### Encoders

docs/sidebars.js

@@ -56,7 +56,10 @@ module.exports = {
         type: "category",
         label: "Guides",
         collapsed: false,
-        items: ["development/new-shield"],
+        items: [
+          "development/new-shield",
+          "development/hardware-metadata-files",
+        ],
       },
     ],
   },