Add GitHub Actions workflow for Jekyll deployment

[smlambert]

Fixes #7139
This workflow builds and deploys a Jekyll site to GitHub Pages, allowing for manual triggers and concurrent deployments.

[smlambert]

@copilot - for any uses statements use a specific SHA of the action rather than refering to a version tag

[Copilot]

@copilot - for any uses statements use a specific SHA of the action rather than refering to a version tag

Implemented in 3bb6b77. All uses: references in .github/workflows/jekyll-gh-pages.yml are now pinned to specific commit SHAs.