Docs: Improve Presets Concept Page Clarity

[!NOTE] Triage estimate — cycle priority and story points were assigned during an AI-assisted planning pass and are rough first guesses. Confirm or adjust when you pick this up.

Context

The presets concept page opens with a technical definition and then describes the /background preset's contents. A fresh reader may struggle to understand what presets are and when they'd reach for one.

Feedback from @davekiss:

I read the intro section for "Presets" and i'm still having trouble understanding what it actually is. may wanna take a pass on that for clarity. For the example that you use, you're describing the background preset and what it includes, but you might be better off describing the user's situation or use case that they are finding themselves in and then what preset they might reach for to solve for that use case.

Notes

Presets are a complicated topic — the current framing may just need a rewrite pass to lead with user scenarios rather than technical internals.