feat(build): Explicit missing keymap node error

* Explicit error if zmk,keymap not set
* Document keymap error

---------

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

app/src/keymap.c

@@ -31,6 +31,12 @@ static uint8_t _zmk_keymap_layer_default = 0;
 
 #define DT_DRV_COMPAT zmk_keymap
 
+#if !DT_NODE_EXISTS(DT_DRV_INST(0))
+
+#error "Keymap node not found, check a keymap is available and is has compatible = "zmk,keymap" set"
+
+#endif
+
 #define TRANSFORMED_LAYER(node)                                                                    \
     { LISTIFY(DT_PROP_LEN(node, bindings), ZMK_KEYMAP_EXTRACT_BINDING, (, ), node) }
 

docs/docs/troubleshooting.md

@@ -40,6 +40,10 @@ West build errors usually indicate syntax problems in the `<keyboard>.keymap` fi
 If you are reviewing these errors in the GitHub Actions tab, they can be found in the `West Build` step of the build process.
 :::
 
+#### Keymap error
+
+If you get an error stating `Keymap node not found, check a keymap is available and is has compatible = "zmk,keymap" set` this is an indication that the build process cannot find the keymap. Double check that the `<keyboard>.keymap` file is present and has been discovered by the build process. This can be checked by looking for a line in the build log stating `-- Using keymap file: /path/to/keymap/file/<keyboard>.keymap`. Inside the keymap file ensure the keymap node has `compatible = zmk,keymap` and it's not misspelled. For more information see the [Keymap](features/keymaps.mdx) and [Config](config/index.md) documentation.
+
 #### devicetree error
 
 A `devicetree error` followed by a reference to the line number on `<keyboard>.keymap` refers to an issue at the exact line position in that file. For example, below error message indicates a missing `;` at line 109 of the `cradio.keymap` file: