Add [package.metadata.binstall] to crates/pdftract-cli/Cargo.toml to enable
cargo binstall to download pre-built binaries from GitHub Releases instead
of compiling from source. Also add comprehensive Installation section to
README.md documenting cargo binstall as the recommended install method.
Bead: pdftract-1u80
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
