Reorganize site to group related content together.

[rossbar]

Thanks to the excellent contributions from the community, there are now ~10 notebooks in the repo. The notebooks naturally fall into different categories: some are more focused on numpy features (like the masked array or SVD tutorial) whereas others highlight applications using NumPy. There are also some meta-notebooks that deal with contributing, as noted in #85.

This PR introduces a hierarchical organization based on these categories to make navigating the notebooks repo easier. This can also help with cross-linking certain tutorials with the main documentation (see numpy/numpy#19191).

Also closes #85

[melissawm]

Hi @rossbar thanks for this! Looks great.

I left one comment that we can think about - otherwise it's a thumbs up from me.

[melissawm]

Thanks, @rossbar !