docs: mention ion item group ESLint rule

[rdlabo]

Summary

• add the Japanese Angular ESLint guidance to the iOS 26 guide
• add the same guidance to the MD3 guide
• link both pages to the require-ion-item-group rule reference

Related English source updates:

• docs: mention ion item group ESLint rule ionic-theme-ios26#123
• docs: mention ion item group ESLint rule ionic-theme-md3#28

Checks

• Prettier check
• git diff --check