feat: Verify steps checker

Create a course checker which will verify that the user has specified an
- `intro.md`
- `step*.md`
- `finish.md`

This will exclude if they decide to add an intro and finish the details section in the metadata.

Start by locating the course checker or validation entry point and inspect how course metadata and files are currently read. Verify the expected presence of intro.md, step*.md, and finish.md, including the stated metadata exclusion case; done means the checker reliably distinguishes valid and invalid courses.

Written by the indexing model from the issue text.