Add GitHub Pages documentation site with orchestrated workflows

This commit creates a comprehensive documentation publishing system that
combines coverage reports, Python API docs, and project documentation into
a unified GitHub Pages site.

Changes:
- Update coverage-nightly.yaml to support workflow_call trigger
- Update python-docs-nightly.yaml to support workflow_call trigger
- Add docs-nightly.yaml workflow for project documentation
- Add publish-site.yaml orchestrator workflow

The publish-site workflow:
- Triggers all three documentation workflows in parallel
- Collects artifacts from each workflow
- Organizes them into a unified site structure:
  - / (root) - Beautiful landing page with navigation
  - /docs/ - Project documentation
  - /python/ - Python API reference (pdoc3)
  - /coverage/ - Code coverage reports
- Deploys to GitHub Pages with proper permissions
- Runs nightly at 5 AM UTC, after all other workflows complete

Each workflow can be:
- Triggered manually via workflow_dispatch
- Called from other workflows via workflow_call
- Run on schedule (coverage: 2 AM, python-docs: 3 AM, docs: 4 AM)

The site includes:
- Modern, responsive landing page with gradient design
- Navigation cards for each documentation section
- 404 error page
- .nojekyll file to prevent Jekyll processing
- Automatic timestamp updates

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

Co-Authored-By: Claude <noreply@anthropic.com>
Changelog-None

.github/workflows/python-docs-nightly.yaml

@@ -6,6 +6,8 @@ on:
     - cron: '0 3 * * *'
   # Allow manual triggers for testing
   workflow_dispatch:
+  # Allow being called from other workflows
+  workflow_call:
 
 concurrency:
   group: python-docs-${{ github.ref }}