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

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(dot)g(dot)johnston(at)gmail(dot)com>
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/173737973383.1070.1832752929070067441@wrigleys.postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4907ba304c346051a6535e67c043779755a78e84

Modified Files
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doc/src/sgml/advanced.sgml | 41 +++++++++++++++++++-----------------
src/tutorial/advanced.source | 49 ++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 71 insertions(+), 19 deletions(-)