Update docs for new executors

[ekzhu]

Why are these changes needed?

Update docs to replace legacy executors with new executors.

Related issue number

Closes #2080

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 37.06%. Comparing base (a8abe59) to head (7956806).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2119   +/-   ##
=======================================
  Coverage   37.06%   37.06%           
=======================================
  Files          68       68           
  Lines        7069     7069           
  Branches     1543     1543           
=======================================
  Hits         2620     2620           
  Misses       4218     4218           
  Partials      231      231           

Flag	Coverage Δ
unittests	37.04% <ø> (ø)

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