Remove `app_root` Setting

[Oglopf]

Looks like we have some functionality in the docs that doesn't work and has been broken for a while.

The issue in the code is here: OSC/ondemand#782

And the portion of the docs to remove is here: https://osc.github.io/ood-documentation/latest/reference/files/nginx-stage-yml.html?highlight=app_root

So let's either remove this for now until the patch gets in or prioritize the fix.