doc: drop almost all bits related to installation

Summary:
In the majority of the cases, users install krusader as available in
their distribution, and thus most of this content is not useful; users
that want to compile krusader from sources find already instructions on
krusader.org and in the in-source INSTALL files, so no need to
duplicate them in a user-taylored documentation. Just link to the
online page on krusader.org.

Furthermore:

• instructions related to building distro package are definitely not in the scope of an user documentation
• the bits about krusaderui.rc are wrong, since this is is actually a bug in krusader (i.e. krusaderui.rc was changed without bumping its version)
• links to distro packages (including macOS and Windows ones) either are obsolete, or simply not useful
• the reference of the command line options is not useful for general users starting krusader graphically; for console users there is already the krusader man page with these bits of information (so no need to duplicate them)
• the instructions for changing the default file manager are obsolete

Test Plan:

• build the documentation
• verify the mentioned bits are not present

Reviewers: yurchor

Reviewed By: yurchor

Subscribers: yurchor, kde-doc-english

Tags: Documentation

Differential Revision: https://phabricator.kde.org/D20287