Extend process_notebooks for testing

[jackgerrits]

Why are these changes needed?

• Enables process_notebooks to test notebooks too
• Also removes the manual quarto render step

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 35.60%. Comparing base (4d0d486) to head (6d6d299).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1789      +/-   ##
==========================================
- Coverage   35.69%   35.60%   -0.10%     
==========================================
  Files          63       63              
  Lines        6625     6625              
  Branches     1454     1454              
==========================================
- Hits         2365     2359       -6     
- Misses       4067     4073       +6     
  Partials      193      193              

Flag	Coverage Δ
unittests	35.60% <ø> (-0.10%)	⬇️

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.