Add extra optional-dependencies section for documentation requirements

[sjvrijn]

Description

• [x] I have read the contribution guidelines
• [x] This update is in line with what is recommended in the Python chapter of the Guide
• [x] All user facing changes have been added to CHANGELOG.md

Create a separate optional-dependencies section for documentation, and switched .readthedocs.yaml from 'dev' to 'docs'.

No documentation dependencies can be removed from 'dev', since we also have tests on the documentation generation.

Fixes #317