chore: add more content

[hansemannn]

This pull request updates the introduction and highlights section of the Titanium SDK 13.0.0 RC release notes to better communicate the major new features and improvements in this release. The changes make the document more informative and user-friendly, especially by emphasizing iOS 26 and Android 16 KB page size support, and by listing key new APIs and enhancements.

Release highlights and documentation improvements:

• Added a new highlights section summarizing major new features, including full support for iOS 26/Xcode 26, the new Glass Effect API, bottomAccessoryView for tab groups, modern button configuration, improved option dialogs, and enhanced safe area handling on iOS, as well as compliance with Android's 16 KB page size requirement.
• Improved the clarity and formatting of the introductory paragraph, including correcting "16kb" to "16 KB" for consistency.
• Added a note of appreciation to the community and a call for support, as well as a statement highlighting Titanium's leadership in supporting new iOS 26 features.