Documentation site under docs/content/docs/architecture/ #46

Goal

Turn docs/design.md into navigable documentation, mirroring
specsnl/specs-cli.

The design document is a design document — it records reasoning, alternatives considered, and open
questions. Users need something else: how to write a config, what the modes do, what the exit codes
mean. Both should exist, cross-linked, not merged.

Design reference

docs/design.md § Repository scaffolding,
§ Package structure

Scope

• docs/content/docs/architecture/overview.md and library-decisions.md
• A configuration reference covering groups, labels, renames and defaults
• A getting-started page leading with export, because descriptions are authoritative and a
  naive first run wipes them
• Rewrite README.md — it is currently one line
• Document the exit codes and the CI recipe, including that the Actions-injected
  GITHUB_TOKEN cannot write labels to other repositories
• Keep docs/design.md as the design record and link it from the architecture pages
• task md:check passes

Depends on

#15 (for the docs/ skeleton). Otherwise parallel with all implementation work.