Update and polish the sphinx docs about procrustes

[FanwangM]

The HEADER in the docs folder are not updated yet and I will fix it soon following 06d2c28. I will not touch the docstrings and other parts in the PY files for now until you tell me to.

This is just a reminder for me to double-check before the final release. I will only touch these files located in docs, not others.

Thanks for reminding the potential conflicts I may cause. @FarnazH

[FarnazH]

@fwmeng88 what do you want to change in the HEADER? There were updated in 06d2c28

Also, I am making changes to the docstrings (and equations), and to avoid conflicts, it's best not to change equations now. Then, we should make the paper's appendix consistent with the docstring.

I am also taking care of issue #25, as I messaged you.

[FarnazH]

@fwmeng88 can you please add Table 1 and Equation 1 (from the paper) to the documentation (1st page)? We need to make the Procrustes method names clickable so they link to the API documentation. Thanks.

[FanwangM]

I will do it shortly.