The [[https://doc.wikimedia.org/mediawiki-core/master/php/group__FileBackend.html#intro | generated MediaWiki PHP documentation ]] has an Introduction section. All the sections under this in the left-hand nav (Introduction > Introduction, Setup, File Operations...) come from includes/filebackend/README. The intent in the README markup is they should appear in a separate File Backend section:
```
/*!
\ingroup FileBackend
\page file_backend_design File backend design
```
The same material appears under Modules > File backend in the left nav if you scroll down to File backend design, and there's a link to it when you click [[ https://doc.wikimedia.org/mediawiki-core/master/php/html/group__FileBackend.html#details | More...]] in the introduction to Modules > File backend. So just discourage the material from appearing in the top-level Introduction as well.