docs: fix primary key in PG migration

When importing data, Postgres doesn't update the auto increment counter for the groups. Creating a group after an import would fail due to duplicate IDs. This manually sets the ID to the max of the IDs + 1.

docs/database_migration.md

@@ -53,6 +53,7 @@ format to PostgreSQL format, and wrap it all in a transaction:
 sed -i -r -e "s/X'([[:xdigit:]]+'[^'])/'\\\x\\1/g" \
 -e ":a; s/(INSERT INTO user_attribute_schema\(.*\) VALUES\(.*),1([^']*\);)$/\1,true\2/; s/(INSERT INTO user_attribute_schema\(.*\) VALUES\(.*),0([^']*\);)$/\1,false\2/; ta" \
 -e '1s/^/BEGIN;\n/' \
+-e '$aSELECT setval(pg_get_serial_sequence('\''groups'\'', '\''group_id'\''), COALESCE((SELECT MAX(group_id) FROM groups), 1));' \
 -e '$aCOMMIT;' /path/to/dump.sql
 ```