Add anthropic docs

[victordibia]

Why are these changes needed?

Add anthropic docs

• Add api docs
• Add sample code + usage in agent chat user guide

Related issue number

Closes #5856

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 75.68%. Comparing base (704dab1) to head (f843b6d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5882   +/-   ##
=======================================
  Coverage   75.68%   75.68%           
=======================================
  Files         189      189           
  Lines       12772    12772           
=======================================
  Hits         9666     9666           
  Misses       3106     3106           

Flag	Coverage Δ
unittests	75.68% <ø> (ø)

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