Add a step by step guide

[moorepants]

Good that you explained all the code thoroughly, all the python is clear now.

During reading I sometimes find it hard to know at what step I am of the process. For you it is obvious what steps you need to take to reach the goal and in which order. I think it would be very useful to have a summarized/general 'step plan' for the whole process from problem to answer. such as:

Generalized/independent speeds
Frames
declare positions/speeds of important points in terms of gen. coord/speeds
...
numerical integration
simulation

Maybe this could be a separate page on the website. "practical guide"

https://github.com/moorepants/learn-multibody-dynamics/pull/75#pullrequestreview-973147650