[docs][Backdrop] Move Backdrop to Utils and clarify it is a low-level utility

[webernir]

Fixes #31371

Scope

Maintainer @mj12albert reframed this on 19 Apr 2026: the original report is a poor/unrealistic demo, not a library ARIA bug. Components that usually use a backdrop (Dialog, Modal) already include it. Typical apps should not use Backdrop directly.

This PR is docs-only. No Backdrop component runtime or ARIA changes.

Changes

• Move Backdrop from Feedback to Utils in the Material UI docs nav (docs/data/material/pages.ts), next to Modal.
• Update the Backdrop page so it describes Backdrop as a low-level utility normally used through Dialog/Modal, and note that the SimpleBackdrop demo is a low-level example.

Why not component ARIA

WCAG 4.1.2 on the standalone Backdrop demo is expected: Backdrop is a dimmed layer, not a dialog. Accessible overlay patterns already live on Dialog and Modal (focus trap, labeling, assistive tech). Adding generic ARIA onto Backdrop would be the wrong layer, which is why the 2023 component approach in #37202 was abandoned.

Test plan

• Docs nav lists Backdrop under Utils, not Feedback
• Backdrop page copy matches the maintainer's utility framing
• No changes under packages/mui-material/src/Backdrop

[code-infra-dashboard]

Deploy preview

• docs/data/material/components/backdrop/backdrop.md

  
  
• docs/data/material/pages.ts

  
  

Bundle size

Bundle	Parsed size	Gzip size
@mui/material	0B(0.00%)	0B(0.00%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/private-theming	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

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