Add documentation for providers, PDUs and LAUs (#3686)

* Add documentation for providers, PDUs and LAUs

* Updated to clarify usage of external providers + specify England & Wales instead of UK

* Update concepts.html.md.erb

doc/tech-docs/source/concepts.html.md.erb

@@ -62,6 +62,72 @@ The practitioner, one of either the Prison Offender Manager or Community Offende
 
 Important dates related to a person, such as date of release from prison and the end of supervision.
 
+### Provider
+
+A top-level division of the probation service in England and Wales. There are currently 12 geographical providers as
+well as some administrative providers. There are also a set of external providers, which are used for recording cases
+being transferred into and out of the probation service.
+
+Examples are "London" (geographical) or "National Security Division" (administrative).
+
+<!--- The current list is (as of April 2024):
+
+| Code | Name                            | Type           |
+|------|---------------------------------|----------------|
+| N03  | Wales                           | Geographical   |
+| N07  | London                          | Geographical   |
+| N50  | Greater Manchester              | Geographical   |
+| N51  | North West Region               | Geographical   |
+| N52  | West Midlands Region            | Geographical   |
+| N53  | East Midlands Region            | Geographical   |
+| N54  | North East Region               | Geographical   |
+| N55  | Yorkshire and The Humber        | Geographical   |
+| N56  | East of England                 | Geographical   |
+| N57  | Kent Surrey Sussex Region       | Geographical   |
+| N58  | South West                      | Geographical   |
+| N59  | South Central                   | Geographical   |
+| ALL  | No Trust or Trust Unknown       | Administrative |
+| N40  | Central Projects Team           | Administrative |
+| N41  | National Responsibility Divison | Administrative |
+| N43  | National Security Division      | Administrative |
+| CRS  | Commissioned Rehab Services     | Administrative |
+| N21  | External - London               | External       |
+| N27  | External - Wales                | External       |
+| N28  | Ext - Greater Manchester        | External       |
+| N29  | Ext - North West Region         | External       |
+| N30  | Ext - West Midlands Region      | External       |
+| N31  | Ext - East Midlands Region      | External       |
+| N32  | Ext - North East Region         | External       |
+| N33  | Ext - Yorkshire and The Humber  | External       |
+| N34  | Ext - East of England           | External       |
+| N35  | Ext - Kent Surrey Sussex        | External       |
+| N36  | Ext - South West                | External       |
+| N37  | Ext - South Central             | External       |
+| XXX  | ZZ BAST Public Provider 1       | Testing        |
+ -->
+
+### Probation Delivery Unit (PDU)
+
+A further subdivision of a [Provider](#provider). Similarly, these can refer to geographical areas or administrative
+groupings.
+
+Examples are "Doncaster" (geographical), or "Prison offender managers" (administrative).
+
+Legacy services may refer to this as "borough", but Probation Delivery Unit is the correct term to use.
+
+### Local Admin Unit (LAU)
+
+A further subdivision of a [Probation Delivery Unit](#probation-delivery-unit-pdu).
+
+Legacy services may refer to this as "district" or "local delivery unit", but Local Admin Unit is the correct term to
+use.
+
+### Team
+
+A practitioner can be a member of one or more teams.
+
+Each team is linked to a single [Local Admin Unit](#local-admin-unit-lau), and may also have a linked office location.
+
 ## OASys Concepts
 
 Probation concepts as modelled in the OASys application