Updating documentation, removing reference to poetry shell

[dominikgehl]

Fixes #3634

Type of changes

• Bug fix
• New feature
• Documentation / docstrings
• Tests
• Other

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @willmcgugan may be pedantic in the code review.

Description

Only documentation update to remove poetry shell references.

[r-saba]

I ran into this issue as well. poetry shell has been moved to a separate plugin now and can be added using this command poetry self add poetry-plugin-shell

https://github.com/python-poetry/poetry-plugin-shell