fix: avoid installing sphinx-autoapi branch in pyproject.toml

[jorgepiloto]

Fixes the issues raised in https://github.com/ansys-internal/pyworkbench/actions/runs/6343560993 by installing the sphinx-autoapi branch during the CI/CD step.

Changes will need to be reverted in the future once our branch gets merged in sphinx-autoapi.

[Revathyvenugopal162]

merging this PR!