Add sphinx to a poetry dep group

[dskarbrevik]

Changes

• adds a new dep group to pyproject.toml for docs site building.

Notes

• to have this group installed, developers will need to use poetry install --with docs
• I added sphinx v7 to the pyproject because the latset sphinx (v8) uses python =>3.10 but langfair uses python =>3.9 @dylanbouchard for consideration about the value of langfair being inclusive to older versions of python (i.e. python 3.9) and when the right time to upgrade version requirements of our project should happen.