Docs: replace chinese postgres docs urls, update migration step description

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What does this MR do?

Currently the registry metadata database migration instruction contains chinese links to the PostgreSQL docs that can't be resolved outside from China. This MR replaces the links with the official postgresql.org links.

Also I clarified the actual migration step for the one-step migration approach, as it is currently not visible where the migration will be done and that you have to wait until the migration finishes before updating the chart values again.

Related issues

None

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