docs: guide for merging system.secrets

[vinckr]

Related issue

#2445

As before, all credit goes to @bnm12.

Proposed changes

Checklist

• I have read the contributing guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security. vulnerability, I
  confirm that I got green light (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further comments

[bnm12]

If there's any way of putting emphasis on this that would be good, it's very important.
This whole manual edits section is very important, but the first part will most likely just fail importing and not cause any harm if not done correctly, and can be recovered with a correct import afterwards (with a few harmless duplicate key errors). Where as if you forget to remove this line it will reset the sequence of hydra_clients.pk to whatever was in the source DB potentially breaking client creation going forwards until the sequence gets back in sync

[vinckr]

Thanks for the input!
I put in some warnings, to make clear that this can potentially break client creation if you mess it up.