Simplify docs steps in GPU.yml

[abhro]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.
• Any new documentation only uses public API

Additional context

Split up instantiation vs. docs building step, use shell: option.

Also sort of a minor effect pull request to review the docs deploy issue.