Document CRM architecture

Made a good start. Unassigning so others can pick it up until I'm next available.

bluegeek9 → changed the visibility of the branch 1.0.x to hidden.

What I've done is ready for review. I'm sure plenty more could be written.

If what's there is good enough, I propose getting it merged promptly so we can include architecture docs updates in each issue that affects the architecture.

I'm creating a follow-up issue to document and tidy up access control and permissions.

Will need to follow up to document changes from:
📌 Rename Individual to Person Active
📌 Make External Identifiers field's addition optional Active

Do the Diagrams render? The one I included did not.
https://project.pages.drupalcode.org/crm/relationship/

Not sure how to verify whether it will render on GitLab Pages pre-merge (created 📌 Ability to preview GitLab Pages in CI Active ). I do see that your diagram isn't rendering on GitLab Pages. That feels like a GitLab Pages bug, but I can't find anything recent online about it.

https://git.drupalcode.org/issue/crm-3522273/-/blob/3522273-document-crm... shows a syntax error for mine, but I think that's expected because I'm using a GitLab Pages specific syntax to include the diagram and I wouldn't expect it to function at the above URL.

I found https://mkdocs-mermaid2.readthedocs.io/en/latest/superfences/#usage-for-... which proposed a different way to get Mermaid rendering. Might be worth a try.