Linux installers should install manpages to correct location

This isn't working today due to a bug infra that tries to collect manpages from https://github.com/dotnet/sdk/tree/main/documentation/manpages. There are two issues with this: 1) the folder contains more than manpages, i.e. there are tooling that we do not want to package/install, and 2) directory is referenced with incorrect casing and no manpages are found during build.

We are moving to a new model of building DEB and RPM packages, using common Arcade infra, see https://github.com/dotnet/sdk/issues/43688

This new model requires `Manpage` items to be set, in the installer project. However new Arcade infra installs those to `/usr/share/doc`, which is not a correct location. It appears that there are several ways to package and install manpages, and they should most likely be installed ot `/usr/share/man`. This is still TBD.

Related Arcade issue: https://github.com/dotnet/arcade/issues/15243