README: Practice exercise directory structure doesn't show .meta/config.json nor .docs/instructions.md

[petertseng]

https://github.com/exercism/haskell/blob/main/README.md#directory-structure and https://github.com/exercism/haskell/blob/main/README.md#exercise-structure didn't get updated for v3.

• Update both of these to add .meta/config.json and .docs/instructions.md.
• Update the "Exercise structure" heading (and potentially any text) to make clear that the section applies only to practice exercises.
• In your commit message please use a word from https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue to say that the commit closes this issue.