Improve docs

[felixblanke]

This PR bundles work to improve the documentation throughout the code base.

• [X] Split the API reference into multiple pages
• [ ] Add explanation of boundary handling / signal extensions
• [ ] Add documentation of padding mode
• [ ] Add documentation of compatible wavelet objects
• [ ] Extend usage examples
• [ ] Add contribution page
• [ ] Potentially: Introduce release notes?

Further, I would like to go through all modules to check the docstrings for consistency:

• [ ] constants.py
• [ ] continuous_transform.py
• [x] conv_transform_2.py
• [x] conv_transform_3.py
• [x] conv_transform.py
• [x] __init__.py
• [x] matmul_transform_2.py
• [x] matmul_transform_3.py
• [x] matmul_transform.py
• [x] packets.py
• [x] separable_conv_transform.py
• [x] sparse_math.py
• [x] stationary_transform.py
• [x] _util.py
• [x] version.py
• [ ] wavelets_learnable.py

[v0lta]

We have some release notes at https://github.com/v0lta/PyTorch-Wavelet-Toolbox/releases. We could include a link to those in the docs.

[v0lta]

Having these nice improvements dangling here in the PR makes no sense. I will merge these improvements and open an issue to ensure we don't forget the other improvement ideas.

[v0lta]

Okay, it's not that simple, but I think I am getting there.