Fix warnings reported by CI doc-build action

[brycx]

fiat-crypto uses "z[0]" etc to describe its operations in comment. These are intra-docs for Rust, but aren't used as such by fiat-crypto. We should extend the #[allow] for the fiat-crypto module to:

#[allow(
    dead_code,
    non_camel_case_types,
    clippy::unnecessary_cast,
    clippy::unused_unit,
    rustdoc::broken_intra_doc_links,
)]

and surround the two links in the module-level docs for fiat_curve25519_u64 with <link here>, to make the URLs clickable links.