feat: add GitHub stars button to docs navbar and fix footer license

Add a star button with live count from the GitHub API in the top-right
of the docs site. Fix footer from "MIT License" to "AGPLv3 License".

apps/docs/.vitepress/theme/index.ts

@@ -0,0 +1,8 @@
+import DefaultTheme from "vitepress/theme";
+import Layout from "./Layout.vue";
+import "./github-stars.css";
+
+export default {
+  extends: DefaultTheme,
+  Layout,
+};