Add command to create a Swift DocC documentation catalog

[amanthatdoescares]

Description

This change adds a new Swift command that helps users get started with DocC documentation.

The command prompts for a module name and creates a basic DocC catalog in the workspace, including:

• A <Module>.docc directory
• An initial markdown file with the module title

This removes the need to manually set up the DocC folder structure and makes it easier to start documenting Swift packages directly from VS Code.

Issue: https://github.com/swiftlang/vscode-swift/issues/1647

Tasks

• [x] Required tests have been written (not applicable for this change)
• [x] Documentation has been updated (not applicable)
• [ ] Added an entry to CHANGELOG.md if applicable