add complete quaternion example to docs

[rgommers]

There is a quaternion example in https://github.com/xnd-project/gumath/tree/master/libgumath/extending. It's about 100 LoC.

Here is the numpy dtype equivalent: https://github.com/moble/quaternion. It's about 1600 LoC, plus some support code.

It would be good to have a section in the docs called "extending" or similar, that points to quaternion.c, shows how to build it, and use it from Python. This would be a really powerful example.

[rgommers]

@anirrudh will do this in the next couple of weeks somewhere

[rgommers]

Just followed up on this with @anirrudh, this is >50% done and some version of this should be materializing in the next weeks:)