Update vignette, documentation, fix checkbox missingness logic

[rsh52]

Description

This PR clarifies the build_quality_report() documentation and refines how checkbox fields are evaluated. It separates meaningful no-selection states from missing checkbox data and ensures checkbox consistency checks respect form completion status.

Proposed Changes

• Expand documentation for sparse_threshold, including applicable-row calculations, inclusive threshold behavior, and sensitivity implications.
• Document the IQR outlier formula and how outlier_iqr_multiplier affects finding sensitivity.
• Clarify the difference between progress = "auto", "show", and "none".
• Reorganize the quality-report article with dedicated sections for report construction, missingness, and report structure.
• Explain how form status, branching logic, events, and repeating instruments determine missingness applicability.
• Treat explicit all-zero checkbox exports as an observed, meaningful no-selection state.
• Continue treating an all-NA or blank checkbox set as missing on an applicable row.
• Add the field-level informational finding checkbox_no_values_selected when no options are selected across applicable Unverified or Complete rows.
• Restrict checkbox_none_with_other and checkbox_no_values_selected checks to applicable Unverified or Complete forms.
• Document all mutually exclusive checkbox labels: None, No, Not applicable, N/A, NA, and None of the above.
• Update tests to separately cover checked, all-zero, all-missing, Incomplete, and blank-status checkbox states.
• Regenerate mock_quality_report to reflect the revised status rules.

Behavioral Notes

An all-zero checkbox set no longer contributes to missingness. It can produce one informational field-level finding when no options are selected anywhere across assessed rows. This finding is intended as a review prompt because the no-selection state may be valid.

Checkbox values on Incomplete forms or forms with blank completion status are not evaluated. A checked mutually exclusive option is only reported when selected with another option on an applicable Unverified or Complete form.

Validation

• Full package test suite
• Quality-report article rendering
• Full pkgdown site build
• lintr::lint_package()
• air format --check .
• git diff --check