More documentation problems

[lockwo]

While there are still problems with the rendering of the new math ops (see #594), I found what I think is another documentation problem on the tfq website. The descriptions for the attributes of tfq.datasets.spin_system.SpinSystemInfo (https://www.tensorflow.org/quantum/api_docs/python/tfq/datasets/spin_system/SpinSystemInfo) are all missing. This is the only place on the tfq website where the little description boxes have labels but no descriptions (see the attached picture to if this explanation is not clear).

Additionally, since the attribute descriptions exist in the code (see https://github.com/tensorflow/quantum/blob/v0.5.1/tensorflow_quantum/datasets/spin_system.py#L25) I assume this is a bug.

[lockwo]

I just noticed something else too. The tfi_chain page looks as it should

However, the attribute information for the xxz_chain is in code block style instead of attribute style

I would be happy to make a PR to fix these (and the previous issue) if you have any information on how the website is built from the code and how to test is locally.

[lockwo]

Update: the SpinSystemInfo page changed, but is still wrong, now it just says A namedtuple alias for field number 0 for each attribute