MAINT: Documentation for power spectrum

[Fox-Davidson]

Description

Read the Docs not working with new CAMB function deletions, need to remove reference to the function, changed to class This will close #557

Checklist

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