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/api/index.md

@@ -0,0 +1,25 @@
+# API Reference
+
+RoboCo is API-first: the control panel is just a client of the same REST and WebSocket surface you can drive yourself. This section is for integrators and the curious; the live, always-current schema is at **`/docs`** (Swagger UI) and **`/redoc`** on the orchestrator.
+
+<div class="grid cards" markdown>
+
+-   **[REST API](rest-api.md)**
+
+    ---
+
+    The `/api` domain routes, the agent gateway verbs, the error envelope, and where to find the live OpenAPI.
+
+-   **[WebSocket streams](websockets.md)**
+
+    ---
+
+    The `/ws` live streams the panel consumes — per-resource feeds and the operator system stream.
+
+-   **[Authentication](auth.md)**
+
+    ---
+
+    Header-trust mode versus secure token mode, and how the panel stays authenticated.
+
+</div>