Snippets and snippet slices are not documented

[ktoso]

Description

The snippets feature, and snippet slides, introduced in #349

Seem to be missing from any documentation of the project.
If a feature is not documented, it might as well not even exist 😉

Let's fix this by adding documentation about this feature.

Checklist

• If possible, I've reproduced the issue using the main branch of this package.
• This issue hasn't been addressed in an existing GitHub issue.

Expected Behavior

Documentation should reflect the feature set of docc, including the latest snippets :)

Actual behavior

No documentation exists for snippets

Steps To Reproduce

No response

Swift-DocC Version Information

No response

Swift Compiler Version Information

No response

[code-per-day]

Why is this still not documented?