Write and use API specification

[mjgiarlo]

Component parts:

• Create openapi.yml
• Validate openapi.yml in the CI build
• Add OpenAPI validation badge to the README
• Add Committee gem and enable request validation (ideally, also add response validation; if you don't, add the commented-out response validation code and write up a new issue), then fix all the stuff that breaks
• Create docs/ folder (copy artifacts from another repo, e.g., dor-services-app
• Enable GitHub Pages with the docs/ folder as the source (and don't use the Theme Chooser)
• Verify that the automagic OpenAPI documentation works: https://sul-dlss.github.io/workflow-server-rails/