docs: unstructured no longer requires installing detectron2 from source (#5524)
# Update Unstructured docs to remove the `detectron2` install
instructions
Removes `detectron2` installation instructions from the Unstructured
docs because installing `detectron2` is no longer required for
`unstructured>=0.7.0`. The `detectron2` model now runs using the ONNX
runtime.
## Who can review?
@hwchase17
@eyurtsev