Add new lesson explaining mounting and unmounting to main repo

[jdwilkin4]

We had this issue here

freeCodeCamp/freeCodeCamp#61090

They brought up that mounting and unmounting hasn't really been explained. So we should add a lesson on that before this one here

https://www.freecodecamp.org/learn/front-end-development-libraries-v9/lecture-understanding-effects-and-referencing-values-in-react/how-do-you-reference-values-using-refs

That way campers will have the context of what is going on.

We will go with this suggestion too for the new lesson

a discussion of component lifecycle may be needed first if mounting/unmounting are to be referred to