Fix docs gh-pages publish authentication

[JacobSzwejbka]

Summary:

• Stop passing DOC_BUILD_GITHUB_TOKEN into the docs gh-pages publish job.
• Stop overriding origin with that token before pushing generated docs.
• Let the checkout credentials use the job's existing contents: write permission for the gh-pages push.

Failure fixed:

• https://github.com/pytorch/executorch/actions/runs/29237660083/job/86778638704 failed at git push -f with Permission to pytorch/executorch.git denied to GregoryComer.

Testing:

• git diff --check

Notes:

• PyYAML/actionlint are not installed in this local environment, so I could not run a workflow syntax parser locally.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20890

• 📄 Preview Python docs built from this PR

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[github-actions]

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[JacobSzwejbka]

This also fixes https://github.com/pytorch/executorch/actions/runs/29265325297/job/86873583069. That job is still running the old docs publish script with secrets-env: DOC_BUILD_GITHUB_TOKEN and git remote set-url origin ...$SECRET_DOC_BUILD_GITHUB_TOKEN...; the push fails with Permission to pytorch/executorch.git denied to GregoryComer.