Add DocC implementation of current docs

[Mordil]

Motivation

DocC is a fantastic tool that was announced at WWDC 2021, and full support was released with Swift 5.5

Supporting documentation within DocC gives additional capabilities to developers who work with Time to have better symbol linking and usage guides available while offline, and generated to render within Xcode itself.

Changes Made

• Add: Documentation catalog to the Time module
• Add: Documentation articles that copy the text from the existing Documentation/ folder almost verbatim
• Add: Package manifest variant for Swift 5.5 to support DocC symbol production
• Change: Base Package manifest to exclude Documentation catalog

Results

DocC support!

[Mordil]

Here's some screenshots!

[Mordil]

My intention was to just seed the current documentation as DocC for now, leaving the content basically as-is with some early richer linking, and then maybe in a later PR actually editing/re-writing the documentation itself to be more of a first-class citizen as DocC.

I'm not sure how you want to handle the docs at Documentation/.

DocC links (obviously) break when trying to view the markdown in GitHub, so it's a bit substandard to not have a webhosted version of the docs, so I had figured you wanted to keep it as-is until hosting the DocC output