Documentation integration (#4653)

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docs/docs/extend/plugins/label.md

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+---
+title: Label Mixin
+---
+
+## LabelPrintingMixin
+
+The `LabelPrintingMixin` class enables plugins to print labels directly to a connected printer. Custom plugins can be written to support any printer backend.
+
+An example of this is the [inventree-brother-plugin](https://github.com/inventree/inventree-brother-plugin) which provides native support for the Brother QL and PT series of networked label printers.
+
+### Web Integration
+
+If label printing plugins are enabled, they are able to be used directly from the InvenTree web interface:
+
+{% with id="label_print", url="plugin/print_label_select_plugin.png", description="Print label via plugin" %}
+{% include 'img.html' %}
+{% endwith %}
+
+### App Integration
+
+Label printing plugins also allow direct printing of labels via the [mobile app](../../app/stock.md#print-label)
+
+## Implementation
+
+Plugins which implement the `LabelPrintingMixin` mixin class must provide a `print_label` function:
+
+```python
+from dummy_printer import printer_backend
+
+class MyLabelPrinter(LabelPrintingMixin, InvenTreePlugin):
+    """
+    A simple example plugin which provides support for a dummy printer.
+
+    A more complex plugin would communicate with an actual printer!
+    """
+
+    NAME = "MyLabelPrinter"
+    SLUG = "mylabel"
+    TITLE = "A dummy printer"
+
+    def print_label(self, **kwargs):
+        """
+        Send the label to the printer
+
+        kwargs:
+            pdf_data: An in-memory PDF file of the label
+            png_file: An in-memory PIL (pillow) Image file of the label
+            filename: The filename of the printed label (if applicable)
+            label_instance: The Label model instance
+            width: width of the label (in mm)
+            height: height of the label (in mm)
+            user: The user who printed this label
+        """
+
+        width = kwargs['width']
+        height = kwargs['height']
+
+        # This dummy printer supports printing of raw image files
+        printer_backend.print(png_file, w=width, h=height)
+```
+
+### Available Data
+
+The *label* data are supplied to the plugin in both `PDF` and `PNG` formats. This provides compatibility with a great range of label printers "out of the box". Conversion to other formats, if required, is left as an exercise for the plugin developer.
+
+Other arguments provided to the `print_label` function are documented in the code sample above.