Add a command to create a documentation catalog

[award999]

To accompany the docc and markdown documentation preview work for Swift, a new command should be added that walks the user through the generation of a documentation catalog.

[matthewbastien]

Generating a DocC archive is very straightforward. SourceKit-LSP is just looking for a folder within a target's sources that has the .docc extension. So, this command should allow the user to select any target(s) to generate the folder for and then create it. We can also add a markdown file for the module as well which just needs to have the module name of the target as the title:

# ``ModuleName``

[amanthatdoescares]

@award999 I’ve implemented this and will open a PR shortly.