decrease focus on dtypes and move it to later tutorials

[stijnvanhoey]

As suggested by @WillAyd, pandas-dev/pandas#26831 (comment):

think the first few pages make too many references to dtypes, and having worked with a lot of people coming from Excel / Access and to a certain extent SQL backgrounds I think that makes an assumption about data types that not every analyst would have. I'd prefer if we removed anything exposing dtype info (like calls to .dtypes, .info(), etc... or at least deferred it to a discussion later in the getting started