[Feature Request] Move Cores into Categories

Description

Now that work is being done on the Online Updater, would it be possible to finally move cores into their own categories? I brought this up in another issue (#10710) and @hizzlekizzle mentioned that there might be technical reasons that this can't be done currently, but I do see that the content downloads section does have things split into categories already. I'm not sure if there's something different in how these are handled in the backend.

This is one possible structure, but there might be better choices:

• Emulators
  • System Name
    • Core
• Games
• Game Engines
• Media

I'm not totally happy with "Media", so if we can come up with a better name that would be great. "Other" might be another choice, but it's probably best to avoid ambiguous names like that.

@jdgleaver I'm tagging you because you're the one working on all the recent changes. I know you're super busy at the moment, but I wouldn't be surprised if you're already working on this.

The issue names the Online Updater and the content downloads section but no files or tests. Start by locating those components and comparing how their categories are represented, then investigate the mentioned backend constraints and agree on a category structure. Done means cores appear in the chosen categories without breaking existing content downloads.