Improve contributing and installation documentation

[tsalo]

Summary

1. Recommend against installing with pip. We actually haven't pushed to PyPi since 2021.
2. Link to instructions for installing Docker/Singularity.
   • Note expected image size.
   • Note expected memory requirements.
3. Add info about workflow for modifying ASLPrep code and using it in Docker/Singularity.
   1. Fork repo to your account.
   2. Make a branch.
   3. Clone to local machine.
   4. Modify in preferred editor.
   5. Pull unstable ASLPrep Docker/Singularity image.
   6. Run Docker/Singularity image with modified code folder mounted to correct location in image.