ci: Deny `cargo doc` warnings — heartwood

A job should fail whenever rust doc emits warnings.

       export CARGO_HOME=/workspace/deps
       export HOME=/root
       export PATH="/root/.cargo/bin:$PATH"
       export RUSTDOCFLAGS='-D warnings'
       cargo doc --workspace --no-deps

 shell: |
   export RUSTDOCFLAGS='-D warnings'
   cargo --version
   rustc --version