Document external dependencies for building the website

[VojtechStep]

The Rust dependencies we pull in have dependencies on some native tools/libraries, which might not always be installed; I know at least about

• perl; symptom: Can't locate FindBin.pm in @INC at ./Configure when building openssl-sys as a dependency of mdbook-linkcheck
• patch; symptom: Applying "js-tobigint64-overflow.patch"... thread main panicked: Could not apply patches when building libquickjs-sys as a dependency of mdbook-katex