Add pkgdown workflow for GitHub Pages deployment

[soodoku]

Summary

• add a GitHub Actions workflow that builds the pkgdown site and deploys it to the gh-pages branch
• align the pkgdown site URL with the gojiplus GitHub Pages hostname

Testing

• attempted Rscript -e 'pkgdown::build_site()' (fails: Rscript not available in container)

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https://chatgpt.com/codex/tasks/task_e_68d0df6c4594832f93bdd8e089fe0444

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

[P1] Trigger pkgdown workflow on existing main branch

The new workflow only listens for pushes to master, but this repository’s branches are main and work. Because the master branch does not exist here, documentation changes will never trigger a pkgdown build and the site won’t deploy on pushes. Please change the trigger to the actual default branch so the workflow can run.

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