add doc about openai assistants

[qingyun-wu]

Why are these changes needed?

Related issue number

Checks

• 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.

[qingyun-wu]

Needs to check the blogpost link once Gagan's blogpost is published.

[codecov-commenter]

Codecov Report

Merging #659 (a2a2e1a) into main (24807be) will decrease coverage by 5.24%.
Report is 17 commits behind head on main.
The diff coverage is 3.53%.

@@            Coverage Diff             @@
##             main     #659      +/-   ##
==========================================
- Coverage   33.93%   28.70%   -5.24%     
==========================================
  Files          26       27       +1     
  Lines        3194     3383     +189     
  Branches      720      760      +40     
==========================================
- Hits         1084      971     -113     
- Misses       2014     2341     +327     
+ Partials       96       71      -25     

Flag	Coverage Δ
unittests	28.64% <3.53%> (-5.24%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
autogen/agentchat/conversable_agent.py	66.59% <100.00%> (-0.74%)	⬇️
autogen/retrieve_utils.py	0.00% <ø> (ø)
autogen/version.py	100.00% <100.00%> (ø)
autogen/agentchat/contrib/compressible_agent.py	0.00% <0.00%> (-27.33%)	⬇️
autogen/oai/completion.py	15.15% <66.66%> (-0.03%)	⬇️
...gen/agentchat/contrib/retrieve_user_proxy_agent.py	0.00% <0.00%> (ø)
autogen/oai/client.py	22.42% <6.45%> (-18.76%)	⬇️
autogen/agentchat/contrib/gpt_assistant_agent.py	0.00% <0.00%> (ø)

... and 4 files with indirect coverage changes