docs: document theme-mermaid in API sidebar (#8417)

website/docs/api/themes/theme-mermaid.md

@@ -0,0 +1,25 @@
+---
+sidebar_position: 5
+slug: /api/themes/@docusaurus/theme-mermaid
+---
+
+# 📦 theme-mermaid
+
+This theme provides a `@theme/Mermaid` component that is powered by [mermaid](https://mermaid-js.github.io/). You can read more on [diagrams](../../guides/markdown-features/markdown-features-diagrams.mdx) documentation.
+
+```bash npm2yarn
+npm install --save @docusaurus/theme-mermaid
+```
+
+## Configuration {#configuration}
+
+```js title="docusaurus.config.js"
+module.exports = {
+  themes: ['@docusaurus/theme-mermaid'],
+  // In order for Mermaid code blocks in Markdown to work,
+  // you also need to enable the Remark plugin with this option
+  markdown: {
+    mermaid: true,
+  },
+};
+```