Linux installers should install manpages to correct location

[NikolaMilosavljevic]

Installer infra requires Manpage items to be set, in the installer project. However those get packaged and installed to /usr/share/doc, which is not a correct location -

arcade/src/Microsoft.DotNet.Build.Tasks.Installers/build/installer.build.targets

Lines 155 to 157 in a9ef866

	<Copy
	SourceFiles="@(Manpage)"
	DestinationFiles="@(Manpage->'$(_LayoutDocs)/%(RecursiveDir)%(Filename)%(Extension)')" />

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.