Add CMTS Histogram endpoint and visual

- Build PyPNM-CMTS ServingGroup Downstream Histogram Results MkDocs visual using the CMTS family layout
- Group Histogram by Service Group only (no channel grouping for this endpoint)
- Render one combined DS histogram plus 2-up per-CM histogram cards with sysDescr hover tooltips
- Add ServingGroup/Downstream/Histogram requests to PyPNM-CMTS collection (StartCapture, Status, Cancel, Results)
- Use histogram_cmts_operation_id for the CMTS Histogram operation variable flow
- Sync the Histogram Results visualizer script into the CMTS Postman collection
- Regenerate MkDocs visual preview/docs pages and keep sync/docs checks clean  - 2026-02-26 15:26:01

Author: mgarcia01752