Add examples

[qingyun-wu]

Why are these changes needed?

• Fix invalid links
• Make examples discoverable

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 ✅

Comparison is base (7a4ba1a) 27.72% compared to head (be9e2ee) 27.72%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #851   +/-   ##
=======================================
  Coverage   27.72%   27.72%           
=======================================
  Files          27       27           
  Lines        3578     3578           
  Branches      819      819           
=======================================
  Hits          992      992           
  Misses       2514     2514           
  Partials       72       72           

Flag	Coverage Δ
unittests	27.66% <ø> (ø)

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[qingyun-wu]

TODO: add the notebook https://github.com/microsoft/autogen/blob/99227cb5603c6f52b5559bb9df3dab0ac2b44817/notebook/oai_client_cost.ipynb from PR #812 to the Examples page once the PR is merged.