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.

[kilgor-trout-c22]

@OdinakaDico : Thank you for this contribution, the underlying concern is valid and worth addressing: users should know that the HumHub installation directory is the correct document root for their web server.

I decided not to merge this PR as written for a few reasons:

• A :::warning admonition signals potential danger. A user following the guide in order would not encounter this problem, as the existing Apache VirtualHost example already uses the correct path (/var/www/humhub).
• The note only covered Apache, while the same point applies to nginx as well.
• Placing it mid-way through the Apache section is too late, the document root decision is made when the package is first extracted.

We addressed the underlying point by adding a sentence to the "Download and extract" section, where the installation directory is first established. This covers both web server variants without alarming readers who are already on the right track. (see #225)