Remove instructions about outdated KDE4 and replace
them with KDE5 Plasma.

The instructions are rewritten to cover KDE5 specifics
like installing the necessary packages (SDDM).

Happy KDE5-ing on FreeBSD!

PR: 231235
Submitted by: lbartoletti@tuxfamily.org
Reviewed by: kde@, tcberner@
Approved by: tcberner@
Differential Revision: https://reviews.freebsd.org/D18813