docs: complete documentation overhaul

[frack113]

Documentation Overhaul

This PR completes the documentation structure.

Changes

• Add comprehensive documentation structure with Sphinx and Furo theme
• Include Phase 1-5 documentation files (Quickstart, Cheat Sheet, Tutorials, Backend/Transformation Dev guides)
• Fix docstring formatting issues that prevented HTML build
• Configure ReadTheDocs and GitHub Actions for documentation deployment
• Add custom sidebar navigation with Index and Module Index links
• Update pyproject.toml and poetry.lock with Sphinx dependencies

Files Modified

• 21 files changed, 1943 insertions(+), 39 deletions(-)
• New docs: Backend_Development, Conditions_Guide, Quickstart, Transformation_Cheat_Sheet, Transformation_Development, YAML_Pipeline_Tutorial, Changelog
• Config: .readthedocs.yaml, .github/workflows/docs.yml, docs/conf.py, docs/index.rst
• Fixes: sigma/collection.py, sigma/conversion/base.py (docstring formatting)
• Deps: pyproject.toml, poetry.lock (Sphinx dependencies)

Build Verification

• Documentation builds successfully with (0 warnings)
• GitHub Actions workflow configured for automated documentation deployment