If you want the lowest friction, open source, easily extensible Markdown to Web, Kindle, PDF, etc. tool, highly recommend mdBook: https://github.com/rust-lang/mdBook it’s written in Rust, but you don’t have to know any Rust to use it. And then customising is all CSS; for which there are many good (free) themes.
We use it to write docs in our company which are then compiled by GitHub Actions and published as GitHub Pages. The best example of a Book produced with mdBook is the Rust Lang book: https://doc.rust-lang.org/book/
