feat: add doc/index.html documentation for all viewers

[Osayi-ANL]

Issue

The DocumentationDialog infrastructure is in place (src/dashpva/viewer/documentation/dialog.py) and wired into BaseWindow (src/dashpva/viewer/core/base_window.py). It auto-discovers a doc/index.html (or doc/README.md) next to each viewer module and renders it when the user opens the Help menu. However, most viewers have no doc/ folder, so the dialog shows a blank placeholder.

In progress — work started on the documentation branch (worktree at .trees/documentation)

Viewers missing documentation:

• src/dashpva/viewer/area_det/ — Area Detector Viewer
• src/dashpva/viewer/bayesian/ — Bayesian Viewer (README.md exists at module root but not in doc/)
• src/dashpva/viewer/scan_view.py — Scan View
• src/dashpva/viewer/phase_fitter.py — Phase Fitter
• src/dashpva/viewer/pyFAI_analysis.py — pyFAI Analysis
• src/dashpva/viewer/vit_viewer.py — VIT Viewer
• src/dashpva/viewer/launcher/ — Launcher

Already documented:

• src/dashpva/viewer/hkl3d/doc/index.html ✓
• src/dashpva/viewer/workbench/doc/index.html ✓

Fixes

• For each viewer above, create <viewer_dir>/doc/index.html explaining what the viewer does, what each control does, and any non-obvious workflow steps
• For bayesian/: move/rename the existing README.md into bayesian/doc/README.md (or convert to index.html)
• For single-file viewers (scan_view.py, phase_fitter.py, etc.): create a sibling <name>/doc/index.html folder matching what DocumentationDialog._discover_doc_candidates_for() expects

Acceptance Criteria

• Open each viewer and click Help → Documentation (or the documentation action) — a dialog appears with content describing that viewer, not a blank placeholder
• Bayesian viewer: documentation dialog shows content (not "No documentation found")
• Area Detector viewer: documentation explains ROI setup, colormap controls, and mask workflow
• Scan View: documentation explains channel configuration and how to start/stop a scan