style: Center mermaid diagram in RAG Pipelines chapter.

docs/Chapter_12_Retrieval_Augmented_Generation_RAG_Pipelines.md

@@ -930,6 +930,8 @@ log_entry = {
 
 #### Example Secure Ingestion Flow
 
+<div align="center" style="width: 512px; margin: 0 auto;">
+
 ```mermaid
 %%{
   init: {
@@ -964,6 +966,8 @@ graph TD
     linkStyle default stroke:#d4af37,stroke-width:2px,fill:none;
 ```
 
+</div>
+
 ### Regular Security Audits
 
 #### Audit Activities