docs: radical reduction of documentation scope for v6 beta (#1406)

* docs: radical reduction of documentation scope for v6 beta

Archive and basement unreviewed content to ship a focused, minimal doc set.

Changes:
- Archive stale how-to workflow guides (will rewrite for v6)
- Archive outdated explanation and reference content
- Move unreviewed content to basement for later review
- Reorganize TEA docs into dedicated /tea/ section
- Add workflow-map visual reference page
- Simplify getting-started tutorial and sidebar navigation
- Add explanation pages: brainstorming, adversarial-review, party-mode,
  quick-flow, advanced-elicitation
- Fix base URL handling for subdirectory deployments (GitHub Pages forks)

The goal is a minimal, accurate doc set for beta rather than
comprehensive but potentially misleading content.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* refactor: restructure BMM and agents documentation by consolidating and flattening index files.

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: alexeyv