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/Layout.vue

@@ -0,0 +1,22 @@
+<script setup lang="ts">
+import { computed } from 'vue'
+import { useData } from 'vitepress'
+import DefaultTheme from 'vitepress/theme-without-fonts'
+import DocTags from './components/DocTags.vue'
+import DocMetaEx from './components/DocMetaEx.vue'
+
+const { frontmatter } = useData()
+const tags = computed(() => frontmatter.value?.tags ?? [])
+const hasTags = computed(() => tags.value.length > 0)
+</script>
+
+<template>
+  <DefaultTheme.Layout>
+    <template #doc-before>
+      <DocTags v-if="hasTags" style="margin-bottom: 24px;" />
+    </template>
+    <template #aside-outline-after>
+      <DocMetaEx style="margin-top: 24px; padding-left: 16px;"/>
+    </template>
+  </DefaultTheme.Layout>
+</template>