Use GitHub token for documentation deploy

[quinnj]

Summary

• Stop passing the stale DOCUMENTER_KEY to Documenter.
• Use the existing GITHUB_TOKEN with the job's contents: write permission.
• Restore documentation deployment from master.

Root cause

Documenter prefers DOCUMENTER_KEY when it is present. The configured SSH key no longer authenticates, so deployment failed before the valid GitHub token could be used.

Validation

• Workflow YAML parses.
• Documenter selects HTTPS authentication when only GITHUB_TOKEN is present.
• The full local documentation build passes.

Co-authored by Codex

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.25%. Comparing base (b2d89de) to head (fd45bf8).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #57   +/-   ##
=======================================
  Coverage   82.25%   82.25%           
=======================================
  Files           1        1           
  Lines         124      124           
=======================================
  Hits          102      102           
  Misses         22       22           

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