📝 docs(usage): document use_site_for_root parameter (#439)

The `use_site_for_root` parameter was added in #426 but wasn't
documented in the Usage Guide. 📚 Users had no way to discover this
feature when configuring system services running as root that need to
use system-wide directories instead of root's home directory.

Added a dedicated section in the Usage Guide documenting
`use_site_for_root` alongside other parameters like `ensure_exists`,
`multipath`, and `roaming`. The documentation explains the Unix-only
behavior, the default value for backwards compatibility, and includes a
practical example showing how root processes redirect to site
directories.

Fixes #433

Author: gaborbernat