Document custom title animations

[vistar941]

Summary

• Document that custom title views use the opacity animation only.
• Clarify that the iOS 26+ blur and offset effect is limited to string-based title APIs.
• Add concise custom animation examples to the English and Japanese READMEs.
• Explain that passing nil or omitting scrollAwareTitleAnimation(_:) uses the built-in animation for the current OS version.

Impact

Users can now distinguish the behavior of string-based and ViewBuilder-based title APIs and see how to provide or reset a custom animation.

Validation

• Verified the documented modifiers against the public API declarations.
• Checked the Markdown diff with git diff --check.