Add Mermaid diagram support and update architecture documentation

- Included a new JavaScript file for Mermaid configuration to ensure consistent diagram sizing across documentation.
- Enhanced architecture documentation to reflect the updated pipeline engine structure, including VAD, ASR, TD, LLM, and TTS components.
- Updated various sections to clarify the integration of external services and tools within the architecture.
- Improved styling for Mermaid diagrams to enhance visual consistency and usability.

docs/content/javascripts/mermaid.mjs

@@ -0,0 +1,18 @@
+/**
+ * Global Mermaid config for consistent diagram sizing across all docs.
+ * Exposed as window.mermaid so Material for MkDocs uses this instance.
+ */
+import mermaid from "https://cdn.jsdelivr.net/npm/mermaid@11/dist/mermaid.esm.min.mjs";
+
+mermaid.initialize({
+  startOnLoad: false,
+  securityLevel: "loose",
+  theme: "base",
+  useMaxWidth: false,
+  themeVariables: {
+    fontSize: "14px",
+    fontFamily: "Inter, sans-serif",
+  },
+});
+
+window.mermaid = mermaid;