Document how to use pgloader

What does this MR do?

Document how to use pgloader

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/2799

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• 201503ad - WIP, document how to use pgloader

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@stanhu @lbot This is a placeholder and I'll fill versions and commands information whenever we're sure which versions and commands we would be recommending.

On the other hand, the original document also covered migrating from a backup file. While I believe we could still do that, I don't see we could use pgloader on a SQL dump file, so we should probably still restore the SQL dump first, then I see no reason to work with a backup file anymore.

Do you think it's ok we just tell people to restore the backup, and then do the migration? This might take some extra time because restoring a dump might take some time, but I really think we should do the same thing here to avoid schema difference.

What do you think? If it's ok, then I'll drop that section and just state that restore the backup first.

mentioned in issue #3954 (closed)

@godfat @lbot Can we please finish this off?

@stanhu @lbot What do you think we just polish this and merge it anyway? Even if it didn't work well for some particular customers, this information might still be useful for smaller migrations, and this is certainly the way we're moving forward.

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• 201503ad...39dd7736 - 4175 commits from branch master
• 03a484ee - WIP, document how to use pgloader

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unmarked as a Work In Progress

@lbot @axil Please review if this is ok? Thanks! For now I left the version as X.Y.Z and we might want to fill the actual number, or just tell people to use the latest version?

assigned to @lbot