Support `site_url` in great-docs.yml for subdirectory deployments

## The problem I'm hosting my great-docs site on an on-prem server where the site lives at a subpath: ``` http://myserver:3838/data-team/mypackage/ ``` Quarto generates root-relative asset paths by default (`/site_libs/...`, `/reference/...`, etc.). When you're not at the root, every CSS, JS, and navigation link 404s — the site is cooked, looks broken and the entire reference section doesn't show up. The standard Quarto fix is as simpl as setting `website.site-url` in `_quarto.yml`. But since great-docs owns and regenerates `_quarto.yml` on every build, any manual edit gets silently wiped. There's no clean way to persist this. ## What I'm doing now ```bash great-docs build # Patch the generated _quarto.yml before re-rendering python3 -c " import pathlib p = pathlib.Path('great-docs/_quarto.yml') t = p.read_text() if 'site-url' not in t: t = t.replace('website:', 'website:\n site-url: \"http://myserver:3838/data_analytics/mypackage\"', 1) p.write_text(t) " # Re-render with the correct base path quarto render great-docs/ ``` It works, but it feels hacky and it doubles the render time, depends on the `website:` key being formatted a specific way ## The fix A `site_url` key in `great-docs.yml` that gets written into the generated `_quarto.yml` automatically during build: ```yaml # great-docs.yml site_url: "http://myserver:3838/data_analytics/mypackage" ``` That's literally it! FYI anyone not deploying to GitHub Pages or a dedicated domain is likely hitting this. They just might not know why the site looks broken.