Issue Description:

Swift-DocC currently describes every documentation catalog it produces as a “Framework”.

For example, DocC’s own user-facing documentation at https://swift.org/documentation/docc/ is currently incorrectly described as a “Framework”.

While SwiftDocC is a framework, the documentation hosted at https://swift.org/documentation/docc/ is documentation explaining how to use DocC as a tool; it is not documentation of the SwiftDocC framework.

Swift-DocC should be enhanced to allow documentation authors to provide custom text to describe the thing they are documenting. This should allow the DocC user-facing documentation to describe itself as a “Documentation Tool” instead of as a “Framework”. This should also allow for build systems that integrate with DocC to accurately produce documentation for other product types, like executables.