Improve contributor documentation

Description

I am really happy to see that the Typst documentation is made in Typst itself as of 0.15, which I think a testament to how far the language/compiler has come.

I hope to contribute a bit to the documentation myself as I run into areas where it is a little too sparse. As I got set up for #8686, I noticed that the documentation build system doesn't appear to be documented anywhere (not in CONTRIBUTING.md nor in architecture.md) is clearly documented in main/docs as pointed out by @cady-b.

I started adding a section to CONTRIBUTING.md in a branch, but thought it didn't fit that well with the rest of the document.

I am wondering if there would be appetite for a section of the documentation on the website dedicated to the both a basic development setup and/or a setup for people just wanting to contribute to the documentation, or alternatively whether there would be interest in a PR that terse documents the typst-docs command in CONTRIBUTING.md?

Start by comparing CONTRIBUTING.md and architecture.md with the documentation in the main/docs directory, especially the existing material around the documentation build system. Clarify whether the intended result is a website section covering development and documentation setup or a concise explanation of the typst-docs command in CONTRIBUTING.md; done means one agreed contributor-documentation path is added without duplicating existing guidance.