[docs] Template moulding in docs/platform and index improvement (#3663)

docs/templates/concept_guide_template.mdx

@@ -45,11 +45,11 @@ icon: "lightbulb"
 - **[Term]** – [Short definition]
 - **[Term]** – [Short definition]
 
-<!-- Optional: delete if not needed -->
+{/* Optional: delete if not needed */}
 ```mermaid
 graph LR
-  A[Input] --> B[Concept]
-  B --> C[Outcome]
+  A[Input] */} B[Concept]
+  B */} C[Outcome]
 ```
 
 ## How does it work?
@@ -91,7 +91,7 @@ graph LR
 - [Cookbook or integration demonstrating the concept]
 - [Recording, demo, or sample repo]
 
-<!-- DEBUG: verify CTA targets -->
+{/* DEBUG: verify CTA targets */}
 
 <CardGroup cols={2}>
   <Card