Maskformer post-processing fixes and improvements
# What does this PR do?
- Improves MaskFormer docs, corrects minor typos
- Restructures `MaskFormerFeatureExtractor.post_process_panoptic_segmentation` for better readability, adds target_sizes argument for optional resizing
- Adds `post_process_semantic_segmentation` and `post_process_instance_segmentation` methods.
- Adds a deprecation warning to `post_process_segmentation` method in favour of `post_process_instance_segmentation` 

Notes:
This PR is part of a larger effort to ensure consistency of post-processing methods across segmentation models, to define common arguments and outputs, and get ImageSegmentationPipeline working with all available models.
- `post_process_semantic_segmentation` returns segmentations as tensors of shape (height, width), which is consistent with the COCO format
- `post_process_instance_segmentation` returns segmentations either in the same format as the panoptic method or optionally in run-length encoded format (if return_coco_format is set to True).
- `post_process_semantic_segmentation` currently has an inconsistent input argument (target_size instead of target_sizes) and output (3D tensor instead of list of 2D tensors)




## Before submitting
- [X ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [X ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [X ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


_The documentation is not available anymore as the PR was closed or merged._<|||||>@sgugger @amyeroberts @NielsRogge all comments are addressed, could you approve the PR if everything looks good?