docs: the app can be installed one per cluster

[ra-at-diladele-com]

even if we are able to install two per cluster - as the app itself create kubernetes resources - it is not possible to differentiate what resources we have created. Like:

• admin deploys app1
• admin creates contoso with proxy using app1
• app1 deploys contoso and proxy resources - everything is fine
• admin deploys app2
• admin creates contoso with proxy using app2 this time
• app2 knows nothing about app1 and happily manages resources of app1

Thus we must state that deploying two instances of the app explicitly is NOT supported scenario.