[docs] Add basic documentation on using Snippets to DocC documentation

[heckj]

Work towards resolving #417

Summary

Adds some base line documentation that describes how to use the snippets feature built in to DocC and the DocC Plugin.

• how to add snippets to your project
• how to run and verify the snippets
• how to make slices in a snippet
• how to reference the snippet, or a slice from a snippet, from within another DocC catalog entry

Dependencies & Testing

• no dependencies
• visually inspected with a local preview build of the DocC documentation for testing

Checklist

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

• Added tests (no tests, doc updates only)
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

(this pull request is the combined work of https://github.com/JacobHearst and myself, I just happen to be submitting it)

[d-ronnqvist]

(this pull request is the combined work of https://github.com/JacobHearst and myself, I just happen to be submitting it)

FYI: You can coauthor commits by including a line like this in the commit message:

Co-authored-by: Jacob Hearst <[email protected]>

[heckj]

@JacobHearst @d-ronnqvist sorry for the delay in writing an overview. Added one based on @d-ronnqvist feedback, see what you think.

[heckj]

@swift-ci please test