DNM: Command docs proof-of-concept pitch

[heckj]

DO NOT MERGE

Summary

Extends the DocC user-facing documentation to include content from the command-line controls for DocC.

Dependencies

none.

Testing

• Visual review, a replica of the hosted content with these updates: https://heckj.github.io/swift-docc/documentation/docc/

Steps:

1. check out branch
2. swift package --disable-sandbox preview-documentation --target docc

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

[d-ronnqvist]

FYI: This is very similar to what I started working on in this PR back in February.

[d-ronnqvist]

I converted this to a "draft" to provide extra indication that it isn't shouldn't be merged.