Add database model for defining custom units (#5268)

* Add database model for defining custom units

- Database model
- DRF serializer
- API endpoints

* Add validation hook

* Custom check for the 'definition' field

* Add settings page for custom units

- Table of units
- Create / edit / delete buttons

* Allow "unit" field to be empty

- Not actually required for custom unit definition

* Load custom unit definitions into global registry

* Docs: add core concepts page(s)

* Add some back links

* Update docs

* Add unit test for custom unit conversion

* More unit testing

* remove print statements

* Add missing table rule

docs/docs/concepts/units.md

@@ -0,0 +1,41 @@
+---
+title: Physical Units
+---
+
+## Physical Units
+
+Support for real-world "physical" units of measure is implemented using the [pint](https://pint.readthedocs.io/en/stable/) Python library. This library provides the following core functions:
+
+- Ensures consistent use of real units for your inventory management
+- Convert between compatible units of measure from suppliers
+- Enforce use of compatible units when creating part parameters
+- Enable custom units as required
+
+## Unit Support
+
+Physical units are supported by the following InvenTree subsystems:
+
+### Part
+
+The [unit of measure](../part/part.md#units-of-measure) field for the [Part](../part/part.md) model uses real-world units.
+
+### Supplier Part
+
+The [supplier part](../part/part/#supplier-parts) model uses real-world units to convert between supplier part quantities and internal stock quantities. Unit conversion rules ensure that only compatible unit types can be supplied
+
+### Part Parameter
+
+The [part parameter template](../part/parameter.md#parameter-templates) model can specify units of measure, and part parameters can be specified against these templates with compatible units
+
+## Custom Units
+
+Out of the box, the Pint library provides a wide range of units for use. However, it may not be sufficient for a given application. In such cases, custom units can be easily defined to meet custom requirements.
+
+Custom units can be defined to provide a new physical quantity, link existing units together, or simply provide an alias for an existing unit.
+
+!!! tip "More Info"
+    For further information, refer to the [pint documentation](https://pint.readthedocs.io/en/stable/advanced/defining.html) regarding custom unit definition
+
+### Create Custom Units
+
+To view, edit and create custom units, locate the *Physical Units* tab in the [settings panel](../settings/global.md).