docs(index): add introduction documentation

docs/index.mdx

@@ -0,0 +1,10 @@
+---
+title: Introduction
+description: Welcome to your new Blume docs.
+---
+
+# Introduction
+
+Welcome to **Blume** — markdown-first docs powered by Astro and Vite.
+
+Edit `docs/index.mdx` to get started, then run `blume dev`.