Add Starlight documentation site with full content

Astro 5 + Starlight docs site with 17 pages covering all 30 mcserial
tools. Structured using Diataxis framework: tutorials, guides, reference,
and concepts. Includes Docker + caddy-docker-proxy deployment for
mcserial.l.zmesh.systems with HMR support behind reverse proxy.

Content pages:
- Landing page with feature overview and quick start
- Tutorials: getting started, loopback testing (loop://)
- Guides: RS-232 basics, RS-485/Modbus, file transfers, network ports
- Reference: common tools, RS-232 tools, RS-485 tools, file transfer
  tools, URL schemes, MCP resources, environment variables
- Concepts: RS-232 vs RS-485, flow control

docs/astro.config.mjs

@@ -0,0 +1,76 @@
+// @ts-check
+import { defineConfig } from 'astro/config';
+import starlight from '@astrojs/starlight';
+
+export default defineConfig({
+  site: process.env.PUBLIC_DOMAIN
+    ? `https://${process.env.PUBLIC_DOMAIN}`
+    : 'http://localhost:4321',
+
+  telemetry: false,
+
+  devToolbar: { enabled: false },
+
+  vite: {
+    server: {
+      host: '0.0.0.0',
+      allowedHosts: process.env.PUBLIC_DOMAIN
+        ? [process.env.PUBLIC_DOMAIN, 'localhost']
+        : ['localhost'],
+      ...(process.env.VITE_HMR_HOST && {
+        hmr: {
+          host: process.env.VITE_HMR_HOST,
+          protocol: 'wss',
+          clientPort: 443,
+        },
+      }),
+    },
+  },
+
+  integrations: [
+    starlight({
+      title: 'mcserial',
+      description: 'MCP server for serial port communication — RS-232, RS-485, and file transfers via Model Context Protocol',
+
+      logo: {
+        src: './src/assets/logo.svg',
+        replacesTitle: false,
+      },
+
+      social: [
+        {
+          icon: 'github',
+          label: 'GitHub',
+          href: 'https://github.com/supported-systems/mcserial',
+        },
+      ],
+
+      customCss: ['./src/styles/custom.css'],
+
+      sidebar: [
+        {
+          label: 'Overview',
+          items: [{ label: 'Introduction', link: '/' }],
+        },
+        {
+          label: 'Tutorials',
+          autogenerate: { directory: 'tutorials' },
+        },
+        {
+          label: 'Guides',
+          autogenerate: { directory: 'guides' },
+        },
+        {
+          label: 'Reference',
+          autogenerate: { directory: 'reference' },
+        },
+        {
+          label: 'Concepts',
+          autogenerate: { directory: 'concepts' },
+        },
+      ],
+
+      tableOfContents: { minHeadingLevel: 2, maxHeadingLevel: 3 },
+    }),
+  ],
+});