Add diagram describing the mapping between Form and FormDocument objects

[chao-xian]

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

Rendered: https://github.com/alphagov/forms/blob/add-model-mapping/diagrams/mapping-forms-formdocuments.md

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[stephencdaly]

I don't think this diagram makes a whole lot of sense. There isn't such a thing as a "check condition" or a "goto condition" really. We just have those associations on the page model to get all conditions where the check_page_id or goto_page_id point to the current page as it's sometimes useful to get these. If we want to explain how conditions relate to pages, I think the following is a better way to comprehend it:

The below diagram shows a primary condition (not sure whether we have a better name for these), which skips to Page 3 after Page 1 if the answer given to the selection question on Page 1 matches the answer_value of the condition. For primary conditions, the routing_page_id is always equal to the check_page_id.

flowchart TD
    Form[Form]
    Page1[Page 1]
    Page2[Page 2]
    Page3[Page 3]
    Condition[Condition]

    Page1 --> Form
    Page2 --> Form
    Page3 --> Form
    Condition -->|routing_page_id|Page1
    Condition -->|check_page_id|Page1
    Condition -->|goto_page_id|Page3

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We can also have secondary skip conditions. These cause page(s) to be skipped later on in the form if any answer other than the answer_value was given to the question on the page that the check_page_id references. In this example if any answer other than the answer_value on the primary condition was answered, the user will proceed to Page 2. After answering the question on Page 2, Page 3 will be skipped and they will proceed to Page 4.

flowchart TD
    Form[Form]
    Page1[Page 1]
    Page2[Page 2]
    Page3[Page 3]
    Page4[Page 4]
    Condition[Secondary skip condition]

    Page1 --> Form
    Page2 --> Form
    Page3 --> Form
    Page4 --> Form
    Condition -->|check_page_id|Page1
    Condition -->|routing_page_id|Page2
    Condition -->|go_to_page_id|Page4

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[chao-xian]

TYSM @stephencdaly I'll update with these