Update to Custom Apps documentation

[codefreak558]

Is your feature request related to a problem? Please describe. When I was trying to install the Frappe HR custom module I went through to the custom app wiki page. Built the docker image and all then tried to deploy it without success.

Describe the solution you'd like Add a link to the site operations page where the the text If you need to install some app, specify --install-app. To see all options, just run bench new-site --help. is seen.

Describe alternatives you've considered Another idea is in the custom app's page add that if you add a module that needs to be installed via bench and they are using the pwd.yml file to start there container they can add in the command section of the create-site stack the app they want to install. For example:

bench new-site --no-mariadb-socket --admin-password=admin --db-root-password=admin --install-app erpnext --install-app hrms --set-default frontend;