Update long lists cookbook

[loic-sharma]

Updates the long list cookbook to add ListView.extentItemBuilder, which was added recently for lists with extends of different sizes: https://github.com/flutter/flutter/pull/131393

Presubmit checklist

• [x] This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• [x] This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• [x] This PR uses semantic line breaks of 80 characters or fewer.

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 8dc63e9af5e189de233e8e927a920ef1419ed493):

https://flutter-docs-prod--pr10616-loic-sharma-patch-1-05oncffg.web.app