Fix notebook validation errors for ReadTheDocs build

**Issue**: ReadTheDocs build failing with notebook validation errors
- Missing execution_count fields in code cells
- Missing required notebook metadata

**Solution**:
- Added execution_count: null to all code cells missing this field
- Added missing cell IDs to notebooks without them
- Added standard notebook metadata (kernelspec, language_info)
- Ensured all notebooks have proper nbformat structure

**Files Fixed**:
- All 12 notebooks in docs/source/notebooks/
- AWS, Azure, GCP, Lambda Cloud, HuggingFace Spaces tutorials
- Existing tutorial notebooks (SLURM, PBS, SGE, SSH, Kubernetes)

**Testing**:
- ✅ All 223 tests still pass
- ✅ Documentation builds successfully locally
- ✅ Notebooks now meet nbformat validation requirements

**Script**: Added fix_notebooks.py for future maintenance

This resolves the ReadTheDocs build failures caused by notebook validation.

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

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

Author: jeremymanning