doc: Add example file for COPY

doc: Add example file for COPY

The paragraph for introducing INSERT and COPY discussed how a file
could be used for bulk loading with COPY, without actually showing
what the file would look like.  This adds a programlisting for the
file contents.

Backpatch to all supported branches since this example has lacked
the file contents since PostgreSQL 7.2.

Author: Daniel Gustafsson <[email protected]>
Reviewed-by: Fujii Masao <[email protected]>
Reviewed-by: Tom Lane <[email protected]>
Discussion: https://postgr.es/m/158017814191.19852.15019251381150731439@wrigleys.postgresql.org
Backpatch-through: 13