Add scripts for generating Swift-DocC documentation

[boisy]

Summary

Adds scripts for generating, previewing, and publishing Swift-DocC documentation for CareKit.

Details

• Scripts/generate-documentation will generate a DocC archive containing the documentation for CareKit and emit the path of the archive.
• Scripts/generate-documentation --preview will start a local web server to allow for previewing of the current CareKit documentation.
• Scripts/update-gh-pages-documentation-site will build the documentation for CareKit and deploy it to the gh-pages branch on the remote.
• The script checks the name of the branch it's being run from and automatically creates a separate documentation site per branch. So, moving forward, we'll be able to run the script from the main and stable branches in this repository and have up-to-date documentation for both at separate paths.

Testing

1. Run Scripts/preview-documentation and confirm that the documentation site behaves as expected.
2. Ethan Kusters deployed the documentation to the gh-pages branch on his fork which can be viewed here:
   • https://ethan-kusters.github.io/CareKit/stable/documentation/carekit/
   • https://ethan-kusters.github.io/CareKit/main/documentation/carekit/

Notes

There will be a follow-up PR that adds a DocC catalog to the stable branch of CareKit and properly organizes the project's API.