docs: note requirement of checkout to upload existing files

[zimeg]

Summary

This PR adds a note that the actions/checkout step is required to upload existing contents as a file with the API method technique 📚

From discussion of #448 workarounds!

Requirements

• 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.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 9e415d9

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.86%. Comparing base (1c0bcf0) to head (9e415d9).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #607   +/-   ##
=======================================
  Coverage   99.86%   99.86%           
=======================================
  Files           7        7           
  Lines         736      736           
=======================================
  Hits          735      735           
  Misses          1        1           

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[zimeg]

@lukegalbraithrussell I appreciate what good reading brings thanks for the suggestion 🤓 📚 ✨