docs: Add Vercel note about AI Gateway token needed only locally

[haleychaas]

Summary

Adding a tid bit about only needing the AI Gateway token for a locally running app.
See the preview here: https://docs-slack-d-docs-haley-ow4eor.herokuapp.com/tools/bolt-js/deployments/vercel/

Requirements (place an x in each [ ])

• 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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.44%. Comparing base (f88096a) to head (b4f45ca).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2781   +/-   ##
=======================================
  Coverage   93.44%   93.44%           
=======================================
  Files          37       37           
  Lines        7677     7677           
  Branches      669      669           
=======================================
  Hits         7174     7174           
  Misses        498      498           
  Partials        5        5           

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[zimeg]

🌲 This is a great callout - thanks!!