Add diagram describing the mapping between Form and FormDocument objects

This was pretty confusing and creating this doc helped made it clearer to understand.

Author: chao-xian

diagrams/mapping-forms-formdocuments.md

@@ -0,0 +1,74 @@
+# Form and FormDocument object mappings
+
+## What gets turned into what?
+
+```mermaid
+graph LR
+  Form -- transformed into --> FormDocument
+  Page -- transformed into --> Step
+  Condition -- transformed into --> RoutingCondition
+```
+
+Forms are transformed into Pages, and Pages are transformed into Steps.
+
+Form has a method to transform into a FormDocument called [as_form_document](https://github.com/alphagov/forms-admin/blob/main/app/models/form.rb#L147-L155).
+
+The Page to Step transformation by [this method in the Page](https://github.com/alphagov/forms-admin/blob/main/app/models/page.rb#L94) class.
+
+It basically puts the positional attributes such at the position and ID at the top-level of the step and the other attributes within the "data" field. We add a "type" which for now is always "question_page", but could be "question_set" in the future. This maps mainly onto Option 1 in [this RFC](https://github.com/alphagov/forms/discussions/174) from a while ago.
+
+## How they relate
+
+### Forms
+```mermaid
+graph TD
+  A[Form] -- has many --> B[Page]
+  B <-- has many and belongs to --> C[Condition]
+  D[Routing Condition] -- is a --> C
+  E[Check Condition] -- is a --> C
+  F[Goto Condition] -- is a --> C
+  G[Routing Page] -- is a --> B
+  H[Check Page] -- is a --> B
+  I[Goto Page] -- is a --> B
+  D -- points to --> G
+  E -- points to --> H
+  G -- points to --> I
+```
+
+### FormDocuments
+```mermaid
+graph TD
+  A[FormDocument] -- has many --> B[Steps]
+  B <-- has many and belongs to --> C[Routing Conditions]
+  C -- check_step_id --> B
+  C -- routing_step_id --> B
+  C -- goto_step_id --> B
+```
+
+A FormDocument would include the following:
+
+```json
+{
+     "step": {
+          id: 18
+      /* ..other steps attributes... */
+      "routing_conditions": [
+        {
+          "id": 3,
+          "check_step_id": 18,
+          "routing_step_id": 18,
+          "goto_step_id": null,
+          "answer_value": "Yes",
+          "created_at": "2024-08-02T08:54:07.479Z",
+          "updated_at": "2024-08-02T08:54:07.479Z",
+          "skip_to_end": true,
+          "validation_errors": []
+        }
+      ]
+   }
+}
+```
+
+The `step.id`, is the id of the step (which is a `Page` in active record but stored as `step` in the `FormDocument`).
+
+`step.routing_conditions` contains an array of `Conditions` linked to the `step` through `routing_step_id`. `check_step_id` and `goto_step_id` are references to other `steps` in the form. These are just references, not extra conditions. All Conditions have a `routing_step_id` which links them to a `Step`.