ci(docs): build and deploy the Starlight site to Cloudflare Pages

Adds Node setup, an Astro site build, and a wrangler Pages deploy step
to the existing docs workflow, gated on docs/site/** via the paths
trigger. Also fixes astro.config.mjs's site: URL to match the
fish-config-docs Cloudflare Pages project name (it previously pointed
at fish-config.pages.dev, which is not the project being deployed).

docs/site/astro.config.mjs

@@ -3,7 +3,7 @@ import starlight from '@astrojs/starlight';
 import sidebar from './src/sidebar.json' with { type: 'json' };
 
 export default defineConfig({
-  site: 'https://fish-config.pages.dev',
+  site: 'https://fish-config-docs.pages.dev',
   integrations: [
     starlight({
       title: 'Fish Config',