update instructions for Quickstart

[Borda]

Why are these changes needed?

This simplifies the first steps for a user who wants to try Quickstart and not hit an error...

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md 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]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.22%. Comparing base (84f21a0) to head (39cb562).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7068   +/-   ##
=======================================
  Coverage   81.22%   81.22%           
=======================================
  Files         244      244           
  Lines       18512    18512           
=======================================
  Hits        15036    15036           
  Misses       3476     3476           

Flag	Coverage Δ
unittests	81.22% <ø> (ø)

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