adjust order of side bars

[sonichi]

Why are these changes needed?

Adjust the order of side bars to make it easier to follow.
Improve documentation.
Prepare for new release v0.2.20.

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.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 48.41%. Comparing base (448736e) to head (591276e).

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2040       +/-   ##
===========================================
+ Coverage   36.90%   48.41%   +11.50%     
===========================================
  Files          66       66               
  Lines        7031     7031               
  Branches     1538     1666      +128     
===========================================
+ Hits         2595     3404      +809     
+ Misses       4210     3341      -869     
- Partials      226      286       +60     

Flag	Coverage Δ
unittests	48.28% <100.00%> (+11.37%)	⬆️

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[gagb]

@sonichi https://github.com/microsoft/autogen/pull/2040/files#diff-c2cfa3b1948c53e5e734bd126473aa55c2d09810cc7075bb488963f9639a0aa1 overwrite the formatting fix from #2041. Was this intentional? Sorry for missing this in my review.