Add model specific output classes to PoolFormer model docs
# What does this PR do?
This PR adds 2 model-specific outputs (defined in the `modeling_poolformer.py`) to the PoolFormer docs - `PoolFormerModelOutput` and `PoolFormerClassifierOutput`

## Who can review?
@NielsRogge 
_The documentation is not available anymore as the PR was closed or merged._