Restructure the episode 3 - Data Types and Type Conversion

[greatfog]

It would be more clear to move "Types control what operations (or methods) can be performed on a given value." and "Must convert numbers to strings or vice versa when operating on them." together and move them to the end of the episode. The structure of the 3rd episode will be talking about different data types first, operations of same type of data, then the operations between different data types (floats and integers can be mixed, but string and number couldn't be mixed,) Hope it makes sense.

[alee]

there is some work here in #573 that could be continued