[rush reporter][R10] Publish opt-in reporter docs and demo guide

Parent: #5858

Blocked by: #5978; bootstrap-specific documentation also depends on #5979

Goal

Make the opt-in reporter demo reproducible and document the pre-major safety model.

Document --reporter, --output, --log-level, verbosity aliases, the repository experiment, and RUSH_REPORTER.
Publish a direct rush build demo showing legacy output, compact default output, AI remediation output, and the full-detail log.
Explain TTY/CI behavior, stdout/stderr contracts, color handling, privacy boundaries, and how to disable the feature.
Add bootstrap and cross-version behavior after R6 lands.
Link the migration material from the package/API docs and release notes.

A maintainer can reproduce the demo from a clean checkout using documented commands.
The docs clearly distinguish per-invocation opt-in, repository opt-in, future auto-selection, and Rush 6 defaults.
Emergency rollback and full-log discovery are prominent.
Examples cover interactive, CI, and agent contexts without promising pre-gate auto-selection.

Runs after the demo milestone and contributes to the Rush 6 migration material.

Part of #5858.