Document how to build the website locally

[fredrik-bakke]

There's a make target to install the website dependencies, assuming you have Rust installed (it uses cargo): make install-website-dev. Then to build the website you run make website, and to open it you run make serve-website (it takes a second to start the web server, and IIRC then it should open up the page in your browser automatically).
This should probably be documented somewhere.

Originally posted by @VojtechStep in #622 (comment)