Signup Flows Documentation

A customer asked how it would be possible to avoid some of the Application Plans to be available for Developers to choose upon signup to the Developer Portal, so that only Admins can select those Plans for the Developer, from the Admin Portal.

I found that this is exactly what the Hide button does, in the Application Plans view, but I didn't find it documented, at least in the Signup Flows section [1] or in the Application Plans section [2]. I think it would be a valuable addition to our docs.

It would also be useful to document the availability of the checkbox "Developers can select a plan when creating a new Application" in `[SERVICE] > Integration > Settings`, which is useful in the event the Admin wanted to prevent Developers to select any Application Plan when creating a new Application.

[1] https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.5/html/creating_the_developer_portal/signup-flows
[2] https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.5/html/admin_portal_guide/application-plans