Use Swift Snippets for compilable sample code embedded in docs

We should explore [Swift Snippets](https://github.com/apple/swift-evolution/blob/main/proposals/0356-swift-snippets.md) for our documentation, to ensure they continue to compile successfully even as we evolve the API.

Today, we have to update the snippets in docs manually, and thus they can get out of sync.

Start by reading the linked Swift Snippets proposal and reviewing the repository's documentation snippets. The issue names no files or tests; done means establishing a Swift Snippets-based approach that keeps embedded sample code compiling as the API evolves.

Written by the indexing model from the issue text.