ci(site): add format/lint/test/build workflow for astro site

Add Site GitHub Actions workflow running prettier format check, astro
check, tests, and build on changes under site/. Wire prettier into the
site package with config, ignore, and scripts, and format existing
sources.

site/.prettierrc.json

@@ -0,0 +1,11 @@
+{
+  "plugins": ["prettier-plugin-astro"],
+  "overrides": [
+    {
+      "files": "*.astro",
+      "options": {
+        "parser": "astro"
+      }
+    }
+  ]
+}