Suggest valid categories and stages

Why is this change being made?

The release post item validator checks all the files connect to each other correctly, but the output can be hard to interpret. The output now includes a hint for simple formatting issues, and key/value inversion.

fail data/release_posts/unreleased/jramsay-accept-many-suggestions.yml: invalid category name, 'code_review' name not found in data/categories.yml
  Did you mean 'Code Review'?

fail data/release_posts/unreleased/jramsay-accept-many-suggestions.yml: invalid stage key, 'Create' name not found in data/categories.yml
  Did you mean 'create'?

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what