New README in migrations directory

[i-oden]

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1. Description / Summary

While @rv0lt was working on #1562, he added a README to the migrations directory with some steps regarding how to generate the migrations. There was already some information in the root README, but that file has a lot of other information too. In order to make things clearer and easier to find, I'm now moving the information from the root README to the migrations directory, including some of the information that @rv0lt added, while also including and updating the details that were previously written.

While adding this markdown file, I also linked to the migrations/README.md file from the root README, which made me think of correct/incorrect links, which is why I've also added the Linkspector action.

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• New feature
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  • Non-breaking
• Database change: Remember the to include a new migration version, or explain here why it's not needed.
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• Security Alert fix
  • Package update
    • Major version update
• Documentation
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• Tests only

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 0.00%. Comparing base (a396807) to head (457a3bf).
Report is 24 commits behind head on dev.

Additional details and impacted files

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##   dev   #1576   +/-   ##
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