Clarify what is the MongoDB Swift Driver used for

In the MongoDB Swift Driver page (https://docs.mongodb.com/drivers/swift/) there's no mention why this thing exists, or when we should use this instead of the Realm Swift iOS SDK.

This is mentioned, though, here: https://docs.mongodb.com/realm/sdk/ios/

> Are you looking for information about using Swift with MongoDB in a backend app, command-line utility, or running on macOS or Linux? See the MongoDB Swift Driver documentation.

We should add to the Swift Driver page an Intro paragraph stating exactly that and if you're looking to build an iOS app you should use the iOS SDK.

A/C:

 As a reader, I should understand if I want to use the Swift Driver or the Realm IOS SDK after reading the Docs-Ecosystem Swift page