DOC: Add doctsring for CLASSY class

[Lucia-Fonseca]

Description

This PR adds a docstring for the CLASSY class. Merging this PR closes #561 .

Checklist

• [x] Follow the Contributor Guidelines
• [ ] Write unit tests
• [ ] Write documentation strings
• [ ] Assign someone from your working team to review this pull request
• [ ] Assign someone from the infrastructure team to review this pull request