refactor(playground): update Children Placement page for new autoResize semantics

The infinite growth bug with autoResize + children is fixed (min-height: 0
prevents the feedback loop), so the 'What NOT to Do' warning is no longer
accurate. Updated the page to reflect the new reality:

- Remove outdated 'never use autoResize with children' warning and red cards
- Add autoResize + children demo as the recommended responsive approach
  (children take natural height, chart fills remaining space via flex)
- Retitle AutoSizer example to 'Explicit Control' (still valid but no longer
  the only safe responsive option)
- Extract shared children content into reusable function
- Update sizing strategy guide: autoResize is now recommended, 3-column grid
- Add type='button' to placement toggle buttons
- Simplify page intro and remove GitHub source link

Author: thedanchez