Add ADR to document our use of services

lib/engines/content_block_manager/app/doc/adr/0001-document-use-of-services.md

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+# 1. Document use of services
+
+Date: 2024-01-10
+
+## Status
+
+Accepted
+
+## Context
+
+A long-established pattern in Rails has been to have Ruby classes that sit separate from
+the usual Model, View, Controller approach, carrying out things like business logic, API
+calls etc and putting these in a `app/services` directory. We have used this approach
+for various things in this project, but recently we've noticed that we were a little 
+inconsistent with our approach to services.
+
+Some services carry out business logic (e.g `CreateEditionService`), while others make
+API calls, returning Ruby `Data` classes. 
+
+## Decision
+
+We have decided to continue our use of services for business logic, but if a service
+returns an object, we should have a class-level method on that class to return the 
+object. For example:
+
+### Before
+
+We have a data class called `IceCream`, which looks like this:
+
+```ruby
+class IceCream < Data(:id, :flavour)
+end
+```
+
+And a service method called `GetIceCream`:
+
+```ruby
+class GetIceCream
+  def get_ice_cream_for_van(van_id)
+    api_response = call_some_api_here(van_id)
+
+    IceCream.new(api_response["id"], api_response["flavour"])
+  end
+end
+```
+
+## After
+
+We move all the logic to the `IceCream` class:
+
+```ruby
+class IceCream < Data(:id, :flavour)
+  class << self
+    def for_van(van_id)
+      api_response = call_some_api_here(van_id)
+
+      IceCream.new(api_response["id"], api_response["flavour"])
+    end
+  end
+end
+```
+
+This will then allow us to call `IceCream.for_van(van_id)`, which maps better with
+how database model classes operate in ActiveRecord.
+
+We propose keeping Service classes around, but only in the following circumstances:
+
+- They do not return a model object
+- They carry out business logic which cannot easily be expressed in a model
+- They have a single `call` method, which operates on an instance of the class (eg. `MyService.new(args).call`)
+
+## Consequences
+
+This reduces the risk of the `app/services` dir being a repository for all sorts of different
+types of code and forces us to think about organising our code in a more object-oriented way.