docs: add ai provider token instructions

[zimeg]

Summary

This PR follows up on changes from slack-samples/bolt-python-ai-chatbot#13 to include instructions for gathering tokens from providers 🤖

Testing

🔗 All links remain current!

Category

• Document pages under /docs

Requirements

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run ./scripts/install_all_and_run_tests.sh after making the changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.00%. Comparing base (c512c6b) to head (564cd2e).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1340   +/-   ##
=======================================
  Coverage   91.00%   91.00%           
=======================================
  Files         222      222           
  Lines        7512     7512           
=======================================
  Hits         6836     6836           
  Misses        676      676           

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[zimeg]

@srtaalej Thanks so much for the review! It's so nice to surface these providers together 🤩