Update CHANGELOG with 2.3.1–2.3.6 releases and refine 2.3.0 notes

- Add TextKit2 viewport/layout convergence, safe relayout, and post-layout
  selection scroll handling
- Fix viewport bounds calc, gutter offset/width issues, and ensure viewport
  range layout
- Track speech state via AVSpeechSynthesizer delegate; validate Stop Speaking
- Improve large selection delete performance (#107)
- Resolve NavigationSplitView crash (#102); add ignoresSafeArea; reset typing
  attributes and introduce resetTypingAttributes()
- Fix sizeToFit() gutter width handling (#101)
- Major accessibility improvements (VoiceOver, selection, scrolling, traits,
  notifications) in 2.3.3
- Adjust layout fragment height handling (2.3.2)
- Color/appearance and typing attribute updates (2.3.1)
- Rework 2.3.0 section: consolidate features, fixes, refactors, and tests