cad/gspiceui: Fix documentation path

Improve port:

• Replace PORTVERSION with DISTVERSION.
• Fix warnings from portclippy.
• Remove unnecessary MKDIRs.
• Merge COPYTREE_SHARE.

PR: 295290
Reported by: Nilton Jose Rizzo
Sponsored by: UNIS Labs
Co-authored-by: Vladimir Druzenko <vvd@FreeBSD.org>
MFH: 2026Q2

(cherry picked from commit 9a823ec6ed60849912a65d729ae4d706db0a290c)