Improve documentation for what's the semantics of the content types metadata

For magika up to 0.5.1, the content type metadata included things such as "magic" and "vt_type", which ideally we should document. However, we cleaned most of these things up in magika 0.6.0+, so it's unclear whether we have something to improve on this aspect. Opening this issue as a placeholder until we figure that out.

Review the documentation and the content type metadata semantics for Magika versions up to 0.5.1, including the legacy "magic" and "vt_type" fields. Compare that with the cleanup in Magika 0.6.0+ and determine whether any metadata still needs explanation. Done means the required documentation scope and the semantics to document are clearly settled.

Written by the indexing model from the issue text.