Improve CI Build Performance

[Robinlovelace]

The GitHub Actions build currently takes about 4 minutes and 10 seconds. My goal is to halve this.

Why it takes so long:

1. Full re-render every run: The _freeze/ directory is in .gitignore, so Quarto re-renders all 44+ documents from scratch on every push.
2. Setup overhead:
   • Setting up Quarto takes ~10s.
   • Installing R and system dependencies takes ~1m 20s.
   • Additional Setup (Python) takes ~20s and runs every time without caching (installs jupyter, jupyter-cache).
3. ** Sequential Rendering:** The rendering step takes over 2 minutes.

Proposed solutions:

1. Cache or commit _freeze/: Preserving the _freeze directory will allow Quarto to skip re-rendering unchanged files.
2. Optimize Python setup: Use �ctions/setup-python with caching or a faster installation method.
3. Use a pre-built Docker image: Creating a custom image with all system dependencies (both R and Python) could save over 1 minute of setup time.
4. Parallel Quarto rendering: Enable parallel processing if not already active.