Usage instructions should reflect different platforms

[mattheworiordan]

See https://www.ably.io/documentation/realtime/usage. The Javascript usage instructions are very lightweight and don't refer to platforms like Angular, React, WebPack, React Native etc.

Same applies for things like .NET and Xamarin

We should perhaps at least link to the Github repo for those platforms.

┆Issue is synchronized with this Jira Task by Unito

[mattheworiordan]

We should perhaps update the download page with these other platforms as well.