docs: add the user-facing MkDocs documentation site

Build a complete user-facing documentation site (MkDocs Material) under docs/, served at roboco.dev/docs via a new gh-pages deploy workflow.

- Sections: Get Started, The Company, the Tour, Operating the Panel, Choosing & Running Models, Cost & Observability, Optional Subsystems, Configure & Deploy, API Reference, Troubleshooting & Security (55 pages).
- mkdocs.yml (Material theme; excludes the agent-facing rag/ corpus, internal scratch, and orphaned stub trees) and .github/workflows/docs.yml (mkdocs gh-deploy to gh-pages).
- Retire the stale root usage.md and deployment.md to redirect stubs into the site.
- Fix the docs tooling: add the pymarkdownlnt dependency + .pymarkdown.json, run serve-docs/lint-docs/fix-docs under the docs extra, add a build-docs strict gate.
- Fix the roboco console-script entry point (cli, not the un-awaited async main).
- README: correct the project-structure tree (optimal.py, alembic) and link the docs site.

docs/deploy/index.md

@@ -0,0 +1,31 @@
+# Configure & Deploy
+
+The reference layer beneath [Get Started](../get-started/index.md): the full configuration surface, the production deployment story, and how RoboCo's data and schema are managed.
+
+<div class="grid cards" markdown>
+
+-   **[Deployment](deployment.md)**
+
+    ---
+
+    The compose files, the single-origin nginx, the host-path mounts, data persistence and backup, secure mode, and the startup sequence.
+
+-   **[Environment reference](env-reference.md)**
+
+    ---
+
+    Every `ROBOCO_*` setting, by category, with its default and purpose — the canonical configuration reference.
+
+-   **[Data & migrations](data-and-migrations.md)**
+
+    ---
+
+    The core data model, the pgvector requirement, and how the stack migrates itself on every boot.
+
+-   **[Bootstrap & seeds](bootstrap-and-seeds.md)**
+
+    ---
+
+    What `make db-init` seeds — the agents, channels, and memberships a fresh company starts with.
+
+</div>