docs: add initial Mediary user guide

src/content.config.ts

@@ -0,0 +1,10 @@
+import { defineCollection } from 'astro:content';
+import { docsLoader } from '@astrojs/starlight/loaders';
+import { docsSchema } from '@astrojs/starlight/schema';
+
+export const collections = {
+  docs: defineCollection({
+    loader: docsLoader(),
+    schema: docsSchema(),
+  }),
+};