Add a German macOS installation guide

[Jayktopus]

Goal

Provide a German counterpart to INSTALL_MACOS.md without changing the installation behaviour.

Acceptance criteria

• Add INSTALL_MACOS_DE.md with the same tested steps and warnings.
• Keep shell commands identical to the English guide.
• Link the new guide from README_DE.md.
• Verify all relative links.

This is a documentation-only first contribution.