UI: Add Form best practices Storybook page

[mirka]

What?

Adds a Form Best Practices page under Design System ▸ Components ▸ Form in Storybook.

Why?

People composing several controls under one visible group label need a documented accessible pattern. The Form primitives overview already tells authors to prefer a *Control. This page shows that composition inside a fieldset.

How?

Adds packages/ui/src/form/stories/best-practices.mdx and packages/ui/src/form/stories/best-practices.story.tsx. The example uses Fieldset for the group label and InputControl with hideLabelFromVision for each field, so each input keeps a native label without a second visible one. Hidden labels include the visible unit (lb, pounds) so voice-control users can target the text they see.

Testing Instructions

1. Open Storybook.
2. Go to Design System ▸ Components ▸ Form ▸ Best Practices.
3. Confirm the Shipping weight example shows two number inputs with lb and oz suffixes, and no visible Pounds or Ounces labels.
4. Confirm each input still has an accessible name (lb, pounds, oz, ounces) via its hidden label.

Screenshots

[github-actions]

Size Change: 0 B

Total Size: 7.92 MB

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[github-actions]

Flaky tests detected in 09f89ca.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/33415301078
📝 Reported tests:

two users concurrently move list items (RTC WebSocket) in /test/e2e/specs/editor/collaboration/collaboration-stress.spec.ts, passed after 1 failed attempt.

Error: expect(received).toBeLessThan(expected)

Expected: < 3
Received:   4

Call Log:
- Timeout 10000ms exceeded while waiting on the predicate
    at assertMovedOrder (/home/runner/work/gutenberg/gutenberg/test/e2e/specs/editor/collaboration/collaboration-stress.spec.ts:646:8)
    at /home/runner/work/gutenberg/gutenberg/test/e2e/specs/editor/collaboration/collaboration-stress.spec.ts:650:10

[github-actions]

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Co-authored-by: mirka <0mirka00@git.wordpress.org>
Co-authored-by: ciampo <mciampini@git.wordpress.org>

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[ciampo]

LGTM 🚀

Left one non-blocking comment.

[ciampo]

Nit: The fields show lb and oz, but their accessible names are Pounds and Ounces, so voice-control users cannot reliably target them using the visible text.

Should we include the unit in each label, such as label="lb, pounds", and clarify that labels should only be visually hidden when other visible text still identifies each control?

(related, related)