Add app diagram

[hursey013]

Changes proposed in this pull request:

• Creates mermaid-based diagram of application to support compliance efforts

Submitter checklist

• [x] Added logging is not capturing sensitive data and is set to an appropriate level (DEBUG vs INFO etc)
• [x] Updated relevant documentation (README, ADRs, explainers, diagrams)

Security considerations

Documentation only

[jduss4]

Convenience link to review the diagram:

https://github.com/cloud-gov/cg-ui/blob/da23968a9c52f6f324491dbddbc9dc1fb8726eb7/docs/diagrams/web.md