refactor: remove ToggleButton

[oleksandrzavarzin-callstack]

Motivation

ToggleButton predates the Material Design 3 work and has no place in the v6
component set. MD3 has no equivalent component, and both of its use cases are
already covered by components we keep:

• an icon-only toggle is IconButton with selected
• a set of mutually exclusive options is SegmentedButtons

It is removed outright, with no deprecation alias and no runtime warning, per
the v6 rule against deprecations.

What's removed

• ToggleButton, ToggleButton.Group, ToggleButton.Row, and their utils
• The four public exports: the component plus ToggleButtonProps,
  ToggleButtonGroupProps and ToggleButtonRowProps
• Its test and snapshot
• Its example screen and its entry in the example app's list
• Its entries in docs/component-docs.config.ts and
  docs/src/data/screenshots.ts, which drive 6.x doc generation

Migration

Documented in docs/6.x/docs/guides/migration.md:

- <ToggleButton.Group value={value} onValueChange={setValue}>
-   <ToggleButton icon="format-bold" value="bold" />
- </ToggleButton.Group>
+ <IconButton
+   icon="format-bold"
+   selected={value === 'bold'}
+   onPress={() => setValue('bold')}
+ />

Its line in the guide's animated-style list is gone too, since the component no
longer exists to accept a style.

Notes for review

• Ordering against the Button PR. The migration entry closes by noting that
  Button also gains a selected prop as part of its MD3 update. That prop
  arrives with refactor(button): improve MD3 compliance and modernize content API #4928, so if this lands first the sentence describes a prop not
  yet on main. It reads as forward-looking rather than wrong, but it is worth
  a look if the two land far apart. Every code example in the entry uses only
  APIs that exist on main today.

Test plan

• yarn test, yarn lint, yarn typecheck pass: 54 suites, 669 tests, 1
  skipped, no obsolete snapshots.
• The example app typechecks standalone (tsc -p example --noEmit).
• yarn build in docs/ succeeds, and the built output has no 6.x
  ToggleButton page while the archived older-version pages are unchanged.
• Grepped the tree for any remaining reference: the only ones left are the
  intentional migration entry and the 5.x docs.

---

BREAKING CHANGE: ToggleButton, ToggleButton.Group and ToggleButton.Row
are removed, along with ToggleButtonProps, ToggleButtonGroupProps and
ToggleButtonRowProps. Use IconButton with selected for an icon-only
toggle, or SegmentedButtons for a set of mutually exclusive options.

[JKobrynski]

Following this loses the selected state for screen readers: IconButton's selected only changes colours, while ToggleButton passed aria-selected through. Worth adding aria-selected to the snippet, or having IconButton derive it from selected?

[oleksandrzavarzin-callstack]

@JKobrynski, thank you for the comment! I've updated the migration docs with aria-selected.
As for deriving it from the selected one on the IconButton, I don't think that's in scope for this PR.
I can create a separate PR for it, or we can add it as a part of the IconButton modernization to MD3 (when that PR will be ready)