Add SlicePlaneOpacity extension

[youyouhdhd]

Adds SlicePlaneOpacity, a lightweight 3D Slicer extension for controlling
the opacity and 3D visibility of the standard Red, Yellow, and Green slice planes.

Extension repository:
https://github.com/youyouhdhd/SlicerSlicePlaneOpacity

Key points:

• Pure Python ScriptedLoadableModule
• No third-party dependencies
• Controls only 3D slice-plane model opacity/visibility
• Does not alter 2D foreground/background compositing
• BSD 3-Clause license
• Documentation, icon, overview image, static checks, and basic in-Slicer smoke tests included
• Proposed as Tier 1 (Experimental)

Catalog entry uses schema v1.0.2 and points to the public repository's main branch.

[pieper]

Thanks for the contribution. I wonder if this would make more sense as a module in the sandbox? (@lassoan)