Refactor KanesMethod documentation

[tjstienstra]

Currently, the KanesMethod docstring as well as documentation of Kane's method is partially outdated in terms of bad formatting of some equations as well as difficult to understand. A part of the inspiration for rewriting some of the equations will come from the book of @moorepants.

[moorepants]

This is a relevant PR I made some time back: #11183. I tried to rewrite things there. Maybe be worth looking at.

There are a few things I want to fix in the learn multibody dynamics notation (a few inconsistencies), but it would be nice to make things more consistent here too.

[Vaani-pathariya]

Hello , I am new to sympy but I know python and have an indent towards machine learning .Also I have an interest in physics ,So I would like to contribute to this one.

[tjstienstra]

@Vaani-pathariya feel free to open up a new PR and ask questions if you have any. As mentioned this book and #11183 are two useful resources when working on this issue.