bump version to 0.2.19 and update doc

[sonichi]

Why are these changes needed?

for new release

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 47.82%. Comparing base (c266333) to head (49b60a8).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1971       +/-   ##
===========================================
+ Coverage   37.39%   47.82%   +10.42%     
===========================================
  Files          64       64               
  Lines        6913     6913               
  Branches     1518     1649      +131     
===========================================
+ Hits         2585     3306      +721     
+ Misses       4106     3333      -773     
- Partials      222      274       +52     

Flag	Coverage Δ
unittests	47.69% <100.00%> (+10.29%)	⬆️

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