Add some documentation

[elbotho]

Deleting plugins

example: https://github.com/serlo/db-migrations/blob/main/src/20230917105600-upgrade-all-edtr-io-states.ts#L78-L98

Converting plugins

example: https://github.com/serlo/db-migrations/blob/main/src/20220625202000-convert-important-and-blockquote-to-box.ts#L8-L11