This commit beefs up the package metadata used by the 'cargo deb' tool to
produce a binary dpkg. In particular, we now include ripgrep's man page.
This commit includes a new script, 'ci/build_deb.sh', which will handle
the build process for a dpkg, which has become a bit more nuanced than
just running 'cargo deb'. We don't (yet) run this script in CI.
Fixes#842
