Add documentation for Analytics/Notifications on re-using an existing Pinpoint resource without using the CLI

[abdallahshaban557]

Is your feature request related to a problem? Please describe.
Customers that want to add Pinpoint as an existing resource for Analytics/Notification categories to be used with the Amplify libraries do not have documentation on these steps.

Describe the solution you'd like
Provide documentation on using existing resources for Analytics/Notification categories.

[sebaland]

Amplify Swift has the documentation to set up existing resources:

• Analytics
• Push Notifications

Removing the amplify/ios label

[ankpshah]

Amplify Android has the documentation to set up existing resources:

Analytics
Push Notifications

Removing the amplify/android label