Adds the asset for the manpage to cargo deb config. Also moves the generated manpage file to a .1.gz file. Also, moves back to a build script since that was causing some issues for the automatic Cargo.toml fields detection for manpage and completion generation.
To prevent compilation from happening every time, and only in CI, we use an env var to avoid generation steps.
Adds manpage generation to the build process, as well as following the xtask concept of adding additional build scripts that only need to run on deploy/nightly as opposed to `build.rs`. Note this doesn't follow the recommended method of using workplaces because I don't really want to shift the entire repo structure just for this.
More on xtask: https://github.com/matklad/cargo-xtask
