Add short descriptions of each service to the reference home page

[shepazon]

Describe the issue

The reference home page starts off with a brief introduction, then is just a bare list of each service class in the SDK. It would be extremely helpful to have a short description of each class, perhaps using a definition list format, or presenting the service classes in a table with the first column being the service class link, and the second column a short description. Either format would work and would make identifying the right class much easier.

Links

https://sdk.amazonaws.com/swift/api/awssdkforswift/latest/documentation/awssdkforswift