Improve docs

[felixblanke]

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

• 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
• conv_transform_2.py
• conv_transform_3.py
• conv_transform.py
• __init__.py
• matmul_transform_2.py
• matmul_transform_3.py
• matmul_transform.py
• packets.py
• separable_conv_transform.py
• sparse_math.py
• stationary_transform.py
• _util.py
• 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.