Add custom font styles and theme enhancements

- Created a new CSS file for font imports and defined font families for base, mono, strong, em, and h1 elements.
- Added a new JavaScript file to extend the default VitePress theme, incorporating a custom layout and a Vue component for enhanced functionality.
- Integrated Mermaid diagram support and included the necessary styles for it.

openwiki/theme/index.js

@@ -0,0 +1,16 @@
+import DefaultTheme from "vitepress/theme-without-fonts";
+import { installMermaid } from "v-beautiful-mermaid/client";
+import Viewer from "./components/Viewer.vue";
+import Layout from "./Layout.vue";
+import "v-beautiful-mermaid/style.css";
+import "./fonts.css";
+
+/** @type {import('vitepress').Theme} */
+export default {
+  extends: DefaultTheme,
+  Layout,
+  enhanceApp({ app }) {
+    installMermaid(app);
+    app.component("Viewer", Viewer);
+  },
+};