docs: add markdown documentation structure

[rogerchappel]

What

Proper docs structure in /docs — plain markdown, no static site generator. Ready to point Mintlify, Starlight, or Nextra at this folder when the time comes.

Structure

docs/
  README.md                          # Index with links to all sections
  getting-started/
    installation.md                  # Quick start, Docker, external Postgres
    configuration.md                 # Environment variables
    authentication.md                # Local auth, GitHub OAuth, API tokens
  concepts/
    agents.md                        # Creating and managing AI agents
    tasks.md                         # Task lifecycle and assignment
    skills.md                        # Installing and configuring skills
    inbox.md                         # Priority notifications and approvals
    team-structure.md                # Org charts, delegation, escalation
  guides/
    deploy-first-team.md             # One-click team templates
    connect-providers.md             # Claude, GPT, Gemini, Ollama
    self-hosting.md                  # Production deploy, HTTPS, backups
    budgets-governance.md            # Spending limits, approval gates, audit
  API.md                             # (existing) Full REST API reference
  architecture/                      # (existing) System design specs
  specs/                             # (existing) Feature specs

Also trashed the empty docs-site/ Astro scaffold (was untracked, 0 committed files).