VACMS-15597 Add field_diagram to CMDocument.

[swirtSJW]

Description

Closes #15597

Testing done

Screenshots

QA steps

What needs to be checked to prove this works?
What needs to be checked to prove it didn't break any related things?
What variations of circumstances (users, actions, values) need to be checked?

As an admin

1. Edit an existing Content Model Document.

2. Under the Diagram field fill in the following fields.

   • Title Communication breakdown
   • Mermaid diagram

   sequenceDiagram
    Christian->>+Steve: Hello Steve, how are you?
    Christian->>+Steve: Steve, can you hear me?
    Steve-->>-Christian: Hi, I can hear you!
    Steve-->>-Christian: I feel great!
    Christian->>+Dave: Dave, can you hear me?
    Dave->>+Steve: Can you tell Christian I can hear him.
    Steve->>+Dave: I'm a developer not an telephone operator.
    Dave->>+Christian: I think Steve needs more fiber.
   

   • Caption Dave theorizes that All-bran buds would be a good gift.

3. Save the document

   • Validate that you see the diagram and caption.

Definition of Done

• Documentation has been updated, if applicable.
• Tests have been added if necessary.
• Automated tests have passed.
• Code Quality Tests have passed.
• Acceptance Criteria in related issue are met.
• Manual Code Review Approved.

Select Team for PR review

• CMS Team
• Public websites
• Facilities
• User support
• Accelerated Publishing

Is this PR blocked by another PR?

• DO NOT MERGE

Does this PR need review from a Product Owner

• Needs PO review

CMS user-facing announcement

Is an announcement needed to let editors know of this change?

• Yes, and it's written in issue ____ and queued for publication.
  • Merge and ping the UX writer so they are ready to publish after deployment
• Yes, but it hasn't yet been written
  • Don't merge yet -- ping the UX writer to write and queue content
• No announcement is needed for this code change.
  • Merge & carry on unburdened by announcements

[swirtSJW]

The necessary module has already landed on prod so this can be merged whenever.

[omahane]

Nice. I love the example diagram.
https://www.youtube.com/watch?v=2atkj_KWLl0

[swirtSJW]