docs: update changelog.md, PR template, and CONTRIBUTING.md

[Saransh-cpp]

Description

• The changelog was outdated (we should try updating it with every PR)
• Minor typo in PR template
• Add documentation for nox -s notebooks

Checklist

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't any other open Pull Request for the required change ?
• Does your submission pass pre-commit? ($ pre-commit run --all-files or $ nox -s lint)
• Does your submission pass tests? ($ pytest or $ nox -s tests)
• Does the documentation build with your changes? ($ cd docs; make clean; make html or $ nox -s docs)
• Does your submission pass doctests? ($ xdoctest ./src/ocres or $ nox -s doctests)

Before Merging

• Summarize commit messages into a brief review of the Pull request.