docs: add llms.txt support for LLM-friendly documentation

Add starlight-llms-txt plugin to generate /llms.txt, /llms-full.txt,
and /llms-small.txt files with a quick start guide and usage summary.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: domenkozar

docs/astro.config.mjs

@@ -1,11 +1,37 @@
 // @ts-check
 import { defineConfig } from "astro/config";
 import starlight from "@astrojs/starlight";
+import starlightLlmsTxt from "starlight-llms-txt";
 
 // https://astro.build/config
 export default defineConfig({
+  site: "https://secretspec.dev/",
   integrations: [
     starlight({
+      plugins: [
+        starlightLlmsTxt({
+          description: `SecretSpec is a declarative secrets manager for development workflows. Define secrets in \`secretspec.toml\`, then use the CLI to manage them.
+
+## Quick Start
+
+1. Initialize: \`secretspec init --from .env\` or create \`secretspec.toml\` manually
+2. Set secrets: \`secretspec set DATABASE_URL\`
+3. Check status: \`secretspec check\`
+4. Run commands with secrets: \`secretspec run -- npm start\`
+
+## Configuration Example
+
+\`\`\`toml
+[profiles.default]
+DATABASE_URL = { description = "PostgreSQL connection string" }
+API_KEY = { description = "External API key" }
+\`\`\`
+
+## Providers
+
+Secrets can be stored in: keyring (default), dotenv files, environment variables, 1Password, LastPass, Pass, or Google Cloud Secret Manager.`,
+        }),
+      ],
       title: "SecretSpec",
       logo: {
         src: "./src/assets/logo.png",