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/templates/overview.md

@@ -0,0 +1,21 @@
+# 知识库概览
+
+{知识库描述}
+
+{知识结构逻辑}
+
+## 知识地图
+
+{mermaid流程图}
+
+<!-- 图表中出现节点的描述 -->
+
+- **节点-1**
+  - {描述-1}
+  - {描述-2}
+  - ...
+
+- **节点-2**
+  - {描述-1}
+  - {描述-2}
+  - ...