Migrate logging documentation from swift.org to this repo

Migrate logging documentation from swift.org to this repo: https://www.swift.org/documentation/server/guides/libraries/log-levels.html

Start by comparing the logging documentation at the linked swift.org log-levels page with the documentation structure in this repository. Determine the appropriate destination for the migrated content; done means the logging guidance is available here and covers the source page's material.

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