Update docs to mention need to update PK sequences when importing data to a table

When copying data from a source table to a target table, you may have to modify the table's primary key sequence so that the next insert doesn't violate the primary key constraint, as sequence state isn't carried over with the copy.

For example, if the highest primary key of the pasted data was 42, and the sequence was called "My_pk_seq", with a query such as:

`ALTER SEQUENCE "My_pk_seq" RESTART WITH 43;`

Just thought it might be a good idea to add this caveat to the "Importing and Exporting data in Postico" section of the user guide. Thanks for this great tool!

Open the user guide's "Importing and Exporting data in Postico" section and review how table imports are documented. Add the primary-key sequence caveat and the provided ALTER SEQUENCE example, then verify that the section clearly explains when the sequence must be advanced after copying data.

Written by the indexing model from the issue text.