add RAG under topics

[qingyun-wu]

Why are these changes needed?

add RAG under topics

Related issue number

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• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 37.53%. Comparing base (8f6590e) to head (cfd3563).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1990      +/-   ##
==========================================
- Coverage   37.55%   37.53%   -0.02%     
==========================================
  Files          65       65              
  Lines        6910     6913       +3     
  Branches     1520     1521       +1     
==========================================
  Hits         2595     2595              
- Misses       4090     4092       +2     
- Partials      225      226       +1     

Flag	Coverage Δ
unittests	37.53% <ø> (-0.02%)	⬇️

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[thinkall]

I've pushed a few changes. LGTM now.

Looks like agentchat_qdrant_RetrieveChat.ipynb and agentchat_oai_assistant_retrieval.ipynb need to be updated so that they could be shown in the notebook section in the website. This can be done in a separate PR.

[sonichi]

I'd like to change the tone because we'll revamp RAG. We don't want users to take too much dependency on the current solution, right?