Remove unnecessary ref: master from checkout step

The workflow now only runs from master context (push to master or
workflow_dispatch with ref: master), so explicit ref is redundant.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: laughingman7743

.github/workflows/docs.yaml

@@ -23,7 +23,6 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
         with:
-          ref: master  # Always checkout master for GitHub Pages deployment
           fetch-depth: 0  # Fetch all history for sphinx-multiversion
       - name: Setup Pages
         uses: actions/configure-pages@v5