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/troubleshooting/index.md

@@ -0,0 +1,19 @@
+# Troubleshooting & Security
+
+When something looks stuck, or before you put RoboCo anywhere near a network you don't fully trust, start here.
+
+<div class="grid cards" markdown>
+
+-   **[Common issues](common-issues.md)**
+
+    ---
+
+    The usual snags — agents that spawn but do nothing, a "quiet" provider that's actually parked, an empty knowledge base — with their causes and fixes.
+
+-   **[Security](security.md)**
+
+    ---
+
+    The trust model: header-trust versus secure mode, how secrets and tokens are handled, and the one rule — don't expose it to the public internet.
+
+</div>