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.

.github/workflows/docs.yml

@@ -0,0 +1,42 @@
+# Builds the MkDocs site and publishes it to the `gh-pages` orphan branch.
+# GitHub Pages then serves from that branch (set Settings → Pages → Source:
+# "Deploy from a branch" → gh-pages, and configure the custom domain there).
+name: docs
+
+on:
+  push:
+    branches: [master]
+    paths:
+      - "docs/**"
+      - "mkdocs.yml"
+      - ".github/workflows/docs.yml"
+  workflow_dispatch:
+
+permissions:
+  contents: write
+
+concurrency:
+  group: docs-deploy
+  cancel-in-progress: false
+
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Install uv
+        uses: astral-sh/setup-uv@v5
+
+      - name: Sync docs toolchain
+        run: uv sync --extra docs
+
+      - name: Configure git identity for gh-pages
+        run: |
+          git config user.name "github-actions[bot]"
+          git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
+
+      - name: Build and deploy to gh-pages
+        run: uv run mkdocs gh-deploy --force --strict