feat: add `site_url` to prefix assets

[JohnOlushola]

Description

To support assets (CSS, images, ...) to load properly for incoming rewrites to docs.feo.transitionzero.org, site_url allows specifying where to find those assets

Checklist

• [ ] Dependencies install correctly in a clean environment and code executes;
• [ ] Test coverage extended; created tests fail without the change (if possible);
• [ ] All tests passing;
• [ ] Commits follow a type convention (e.g. https://gist.github.com/brianclements/841ea7bffdb01346392c#type);
• [ ] Extended the README, documentation and/or docstrings, if necessary;

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Name	Status	Preview	Comments	Updated (UTC)
feo-client	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Mar 20, 2024 0:15am