docs(example): demonstrate icon-only mode with custom header

Update the example to showcase the new McpInstallButton features:
- Icon-only mode (no label prop)
- Custom header text

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: scalvert

examples/basic-docs/src/theme/NavbarItem/McpInstallNavbarItem.js

@@ -2,5 +2,12 @@ import React from 'react';
 import { McpInstallButton } from 'docusaurus-plugin-mcp-server/theme';
 
 export default function McpInstallNavbarItem() {
-  return <McpInstallButton serverUrl="https://example.com/mcp" serverName="example-docs" />;
+  // Icon-only mode (no label) with custom header text
+  return (
+    <McpInstallButton
+      serverUrl="https://example.com/mcp"
+      serverName="example-docs"
+      headerText="Install in your AI tool:"
+    />
+  );
 }