fix: use deploy key instead of token for gh-pages deployment

[sakshar2303]

Summary

Fixes #461

Root Cause

The deploy step was using GITHUB_TOKEN which does not have admin access
and cannot bypass branch protection rules on the gh-pages branch.

Changes in javadoc-gh-pages.yml

• Replaced github_token: ${{ secrets.GITHUB_TOKEN }} with
  deploy_key: ${{ secrets.ACTIONS_DEPLOY_KEY }} to allow bypassing
  branch protection rules on gh-pages
• Removed if: github.ref == 'refs/heads/develop' so deployment also
  runs on main
• Removed user_name and user_email as they are not needed with a
  deploy key

[sakshar2303]

please review and merge .