Enable Mermaid diagram rendering in Jekyll site (#483)

Just the Docs theme supports Mermaid natively via config.
This enables ```mermaid code blocks to render as diagrams.

Co-authored-by: adilei <adileibowiz@microsoft.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 3 people

_config.yml

@@ -5,6 +5,10 @@ baseurl: "/CopilotStudioSamples"
 
 theme: just-the-docs
 logo: "/assets/images/logo.png"
+
+# Mermaid diagram support
+mermaid:
+  version: "11"
 favicon_ico: "/favicon.ico"
 
 # Default layout for all pages