Change build system to e.g. Astro

[penelopeysm]

Right now we're using Quarto to build the code this repository into HTML. However, Quarto is primarily a content-driven framework, and specifically, content which involves executing Julia (or other languages') code. In this repo, there is no code, just static content. So there is no real benefit in using Quarto.

In view of this, it would be better to use a framework such as Astro that has the explicit purpose of web design. The benefit of this would be that the code is more composable and maintainable (hopefully). Since Astro can also read Markdown files (https://docs.astro.build/en/guides/markdown-content/) this allows contributors who don't know Astro to still contribute to content.

See: #105 (review)