Add visualization helpers for workforce planning outputs

[rodrigo-arenas]

Goal

Add lightweight visualization helpers that make pyworkforce outputs easier to inspect in notebooks, docs, and operational analyses.

Suggested helpers

• Demand curves: plot forecasted demand by period/day.
• Coverage charts: compare required vs scheduled coverage.
• Shift coverage heatmaps: visualize shifts_coverage arrays.
• Schedule summaries: show resources assigned by shift/day.
• Roster matrices: show named resources vs days/shifts.
• Break timelines: show break placements while highlighting minimum coverage.

Design notes

• Keep helpers optional and lightweight.
• Prefer returning matplotlib axes or pandas-friendly data structures rather than forcing a plotting backend.
• Add small examples and screenshots/docs snippets.
• Include tests for data-shaping helpers even if visual rendering is kept minimal.

User value

These helpers would make examples more compelling, help users validate solver outputs quickly, and improve the documentation for workforce analysts who need to explain plans to stakeholders.