add case studies and proof points

## Summary
Imported from GitHub issue: https://github.com/incursa/workbench/issues/591

## Acceptance criteria
- At least 2 “before/after” snapshots showing how work is captured with and
  without Workbench.
- One end-to-end narrative that traces a work item from creation to PR.
- Documented outcomes or metrics (even qualitative) that reinforce the value.