docs: clarify Apache document root for HumHub setup

[OdinakaDico]

Summary

• Clarifies that packaged HumHub installations should use the HumHub root directory containing index.php as the Apache document root.
• Warns users not to use a generic Yii-style /web document root unless that directory exists in their actual HumHub package.

Evidence

• Based on setup evidence issue IDs S2-I06 and S2-I07, with related routing issues S2-I04 and S3-I02.

Verification

• Confirmed the upstream compare shows one changed file: docs/admin/installation.md.
• Confirmed the final diff is 6 additions and 0 deletions.
• Local Docusaurus build was not run because project dependencies are not installed in this workspace.