Doc: simplify the tutorial's window-function examples.

For the purposes of this discussion, row_number() is just as good
as rank(), and its behavior is easier to understand and describe.
So let's switch the examples to using row_number().

Along the way to checking the results given in the tutorial,
I found it helpful to extract the empsalary table we use in the
regression tests, which is evidently the same data that was used
to make these results.  So I shoved that into advanced.source
to improve the coverage of that file a little.  (There's still
several pages of the tutorial that are not included in it,
but at least now 3.5 Window Functions is covered.)

Suggested-by: "David G. Johnston" <david.g.johnston@gmail.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/173737973383.1070.1832752929070067441@wrigleys.postgresql.org

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