This doc attribute is used by rustdoc when generating documentation
for other crates that depend on this crate. With the html_root_url,
rustdoc will be able to generate correct links into this crate.
See C-HTML-ROOT in the Rust API Guidelines for more information:
https://rust-lang-nursery.github.io/api-guidelines/documentation.html#crate-sets-html_root_url-attribute-c-html-root
A version-sync check was added to ensure that the URL is kept up to
date when the crate version changes.
