Add safe data-sharing guide

[folker]

What changed

• adds a user-facing guide for sharing data within a project, across RCC groups, and outside RCC
• explains primary and supplementary Unix groups, one owning group per file, and membership in multiple groups
• explains directory setgid, its relationship to group write permissions and default ACLs, and safe inspection commands
• recommends a governed /projects/<project>/ area with a shared project group
• documents /homes/<username>/data-for-others/ only as a bounded temporary handoff
• links the guide from Storage and transfer, the reference index, the TL;DR, and both navigation implementations

Why

Users need a clear route from “I need to give someone data” to the correct project membership, filesystem permissions, transfer service, or governed publication mechanism. Unix world-readable permissions and personal home directories are not substitutes for project governance.

Validation

• python tools/validate_repo.py
• custom site build and link check
• 74 repository tests and 6 protected-app tests pass