[docs] Document BASE_UI_ANIMATIONS_DISABLED

[smrifat1411]

Closes #4313.

BASE_UI_ANIMATIONS_DISABLED is used throughout the library's own tests (test/setupVitest.ts) and read in useAnimationsFinished, but it wasn't documented anywhere, so people writing tests against Base UI had no way to find it.

This adds a Disabling animations in tests section to the Animation handbook page covering:

• what the flag actually does — skips the wait for animations to finish, so the deferred work runs immediately
• setting it from a Vitest / Jest setup file
• declaring the global for TypeScript
• setting it in Playwright via addInitScript, since it has to exist before the app scripts run
• the limit: it disables Base UI's internal waiting, not your CSS transitions and animations

Docs-only, no source changes.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@5544
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@5544
  

commit: 12975f0

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	0B(0.00%)	0B(0.00%)

Details of bundle changes

Performance

Total duration: 1,027.76 ms -66.25 ms^(-6.1%) | Renders: 92 ⁽⁺⁰⁾ | Paint: 1,720.44 ms -111.25 ms^(-6.1%)

Test	Duration	Renders
Tooltip mount (300 contained roots)	34.06 ms 🔺+8.98 ms(+35.8%)	10 (+0)

14 tests within noise — details

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