Enhance MongoDB docs with additional tutorials

Update mongodb.adoc to include links to both beginner and intermediate
content for Spring AI and MongoDB integration. Add a new section
"Tutorials and Code Examples" with:

- Link to the Getting Started guide for basic integration
- Link to a detailed RAG tutorial for more advanced usage

This change provides users with a clear path from initial setup to
more complex implementations using the MongoDB Atlas Vector Store.

Author: ashni-mehta

spring-ai-docs/src/main/antora/modules/ROOT/pages/api/vectordbs/mongodb.adoc

@@ -256,5 +256,8 @@ List<Document> results = vectorStore.similaritySearch(
 );
 ----
 
+== Tutorials and Code Examples
+To get started with Spring AI and MongoDB:
 
-If you would like to try out Spring AI with MongoDB, see https://www.mongodb.com/docs/atlas/atlas-vector-search/ai-integrations/spring-ai/#std-label-spring-ai[Get Started with the Spring AI Integration].
+* See the https://www.mongodb.com/docs/atlas/atlas-vector-search/ai-integrations/spring-ai/#std-label-spring-ai[Getting Started guide for Spring AI Integration].
+* For a comprehensive code example demonstrating Retrieval Augmented Generation (RAG) with Spring AI and MongoDB, refer to this https://www.mongodb.com/developer/languages/java/retrieval-augmented-generation-spring-ai/[detailed tutorial].