[PR]: Expand Foundkey migration docs

Created by: Jeder

What does this PR do?

Made Foundkey migration docs clearer, might also be useful for Misskey however I don't use Misskey so I didn't touch that. I have wrote this from memory and experience when doing a test migration run today, so I probably missed out on something while doing this. Some Docker commands might also be redundant, but this is how I do stuff so I did it my way, I guess. Most likely needs interpunction fixes.

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