Add layoutString to the public API

It's a shame that if we're building our own pretty printer that wants nice `Show` instances we have to import from the internal API. What do you think about adding some Doc/SimpleDoc producing functions to the public API?

Start by inspecting the public and internal APIs in the pretty-simple repository and locate the existing layoutString implementation. Determine which Doc or SimpleDoc-producing functions the public API should expose, then verify that a custom pretty printer can use them without importing internal modules. Done means the intended functions are publicly importable and covered by the repository's existing checks.

Written by the indexing model from the issue text.