Snap: improve interface management documentation

The current interface management documentation isn't as a how-to, and isn't clean, clear, or as simple as it could be.

This task is to improve this documentation to make it easier to understand, to make it task-oriented and to either remove any superfluous details, or move them to the explanation section:

How-to: https://snapcraft.io/docs/interface-management

Explanation: https://snapcraft.io/docs/interfaces

Background

Snaps use plugs and slots to allow access to system resources, but the terminology isn't easy to understand for a general desktop audience. This problem is repeated throughout the snap-related documentation.

Prerequisites

You will need some experience using snaps and working with connections and interfaces.

See also Contribute to our documentation.