feat: Add studio related documentation.

* Document setting up studio for a new keyboard definition.
* Document how to enable ZMK Studio for a build, adding reserved layers,
  and controlling which behaviors are built into a studio firmware.
* Document `&studio_unlock` behavior.
* Document studio configuration options.

Co-authored-by: Cem Aksoylar <caksoylar@users.noreply.github.com>

docs/docs/features/studio.md

@@ -0,0 +1,111 @@
+---
+title: ZMK Studio
+---
+
+:::warning[Alpha Feature]
+
+ZMK Studio support is in alpha. Although best efforts are being made, keeping compatibility during active development is not guaranteed.
+
+:::
+
+ZMK Studio provides runtime update functionality to ZMK powered devices, allowing users to change their keymap layers without flashing new firmware to their keyboards. Studio is still under active development, and is not yet ready for casual end user use.
+
+## Building
+
+Building for ZMK Studio involves two main additional items.
+
+- Build with the `studio-rpc-usb-uart` snippet to enable the endpoint used for ZMK Studio communication over USB.
+- Enable the `ZMK_STUDIO` Kconfig setting.
+
+### GitHub Actions
+
+First add a `studio-rpc-usb-uart` to the `snippet` property of your build configuration. For a split keyboard, you should do this _only_ for your central/left side, e.g.:
+
+```
+---
+include:
+  - board: nice_nano_v2
+    shield: corne_left
+    snippet: studio-rpc-usb-uart
+  - board: nice_nano_v2
+    shield: corne_right
+```
+
+Next, enable the `ZMK_STUDIO` Kconfig symbol, for example by adding the following line to your .conf file:
+
+```
+CONFIG_ZMK_STUDIO=y
+```
+
+### Local Build
+
+When building locally, use the `-S` parameter to include the `studio-rpc-usb-uart` snippet. Instead of adding it to your config file, you can also append the `ZMK_STUDIO` Kconfig as an additional CMake argument, e.g.:
+
+```bash
+west build -d build/cl_studio -b nice_nano_v2 \
+  -S studio-rpc-usb-uart -- -DSHIELD=corne_left -DCONFIG_ZMK_STUDIO=y
+```
+
+## Including Unreferenced Behaviors
+
+Normally, ZMK will only build and include behaviors that are referenced by your keymap and unused behavior will be skipped. However, ZMK Studio builds using the `studio-rpc-usb-uart` snippet will automatically define the `ZMK_BEHAVIORS_KEEP_ALL` value, which changes the approach and builds all possible behaviors into the firmware. This lets those behaviors be used in ZMK Studio.
+
+A few controls are available to override this behavior for fine-grained control of what behaviors are built. Behaviors can be kept or omitted by defining certain values in the top of your keymap file, _before_ the standard `behaviors.dtsi` file is included.
+
+By convention, the defines used to keep/omit a given behavior are typically upper-cased versions of the behavior label, e.g. for the `&kt` behavior, the suffix to use would be `_KT`.
+
+### Omit Specific Behaviors
+
+You can omit a specific behaviors by defining a variable like `ZMK_BEHAVIORS_OMIT_KT` at the top of your keymap:
+
+```dts
+#define ZMK_BEHAVIORS_OMIT_KT
+
+#include <behaviors.dtsi>
+```
+
+### Keep Only Selective Behaviors
+
+To override the default "keep all" functionality, you can undefine the `ZMK_BEHAVIORS_KEEP_ALL` flag, and then keep only specific behaviors with a flag like `ZMK_BEHAVIORS_KEEP_KT`, e.g.:
+
+```dts
+#undef ZMK_BEHAVIORS_KEEP_ALL
+
+#define ZMK_BEHAVIORS_KEEP_SK
+#define ZMK_BEHAVIORS_KEEP_MT
+#define ZMK_BEHAVIORS_KEEP_KT
+
+#include <behaviors.dtsi>
+```
+
+## Including Extra Layers
+
+By default, a build with ZMK Studio enabled will only allow as many layers as are defined in your standard keymap. To make additional layers available for use through ZMK Studio, you simply add new empty layers to your keymap with a status of `reserved`, e.g.:
+
+```dts
+/ {
+    keymap {
+        compatible = "zmk,keymap";
+
+        base {
+            display-name = "Base";
+            bindings = // etc.
+        };
+
+        fn_layer {
+            display-name = "Fn";
+            bindings = // etc.
+        };
+
+        extra1 {
+            status = "reserved";
+        };
+
+        extra2 {
+            status = "reserved";
+        };
+    }
+}
+```
+
+The reserved layers will be ignored during regular ZMK builds but will become available for ZMK Studio enabled builds.