Improve Image Generation Notebook

[WaelKarkoub]

Why are these changes needed?

When I first made this notebook, I didn't expect it to be rendered on the website:

• Improved title and descriptions.
• Followed the contribution guidelines closely for consistency.
• Removed caching (I think most users of the notebook are experimenting, so it didn't make sense to cache)
• Fixed bugs

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 37.83%. Comparing base (989c182) to head (20a0428).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2213   +/-   ##
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  Coverage   37.83%   37.83%           
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  Files          77       77           
  Lines        7766     7766           
  Branches     1663     1663           
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  Hits         2938     2938           
  Misses       4579     4579           
  Partials      249      249           

Flag	Coverage Δ
unittests	37.81% <ø> (ø)

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[BeibinLi]

LGTM.

Side note: if we want to render the notebook in the website, then we need to add metadata to the notebook.

See #1495

Maybe we don't need to do that right now, but @jackgerrits can give instructions in the future.

[WaelKarkoub]

@BeibinLi The notebook is currently being rendered (https://microsoft.github.io/autogen/docs/notebooks/agentchat_image_generation_capability/), however, the title and the description didn't make sense in the context of the website, so I just opened this PR to improve that.