docs(config): Add mermaid diagram support (#10357)

# Which Problems Are Solved

#7573

# How the Problems Are Solved

Enabled mermaid support in the current version:
https://docusaurus.io/docs/next/markdown-features/diagrams

# Additional Changes


# Additional Context

test by adding to a page: 

```mermaid
graph TD;
    A-->B;
    A-->C;
    B-->D;
    C-->D;
```

docs/docusaurus.config.js

@@ -23,6 +23,7 @@ module.exports = {
     description:
       "Documentation for ZITADEL - Identity infrastructure, simplified for you.",
   },
+
   themeConfig: {
     metadata: [
       {
@@ -450,9 +451,13 @@ module.exports = {
       };
     },
   ],
+  markdown: {
+    mermaid:true,
+  },
   themes: [
     "docusaurus-theme-github-codeblock",
     "docusaurus-theme-openapi-docs",
+    '@docusaurus/theme-mermaid',
   ],
   future: {
     v4: false, // Disabled because of some problems related to https://github.com/facebook/docusaurus/issues/11040