Swift-Markdown and other Swift-DocC library documentation published to GitHub Pages

Hi all!

I'm excited to announce that the documentation for Swift-Markdown and several other Swift-DocC libraries are now available on GitHub Pages for easy reference.

(Note that Swift-DocC's user-facing documentation continues to be available on Swift.org)

If you're interested in publishing your own framework's documentation to GitHub Pages, check out the Swift-DocC Plugin's documentation, or this PR on Swift-Markdown for a practical example.

Thank you all for the help with this effort, especially to @Kyle-Ye. I'm really excited to have this documentation more generally accessible.

- Ethan