feat: add ci, release notes, and jazzy docs workflow

[vpeschenkov]

Summary

• Add GitHub Actions CI and release notes updates from this branch
• Add Jazzy documentation generation workflow on push
• Run Jazzy through Bundler and lock Ruby dependencies
• Remove Objective-C compatibility remnants: ObjC example app, umbrella header, @objc annotations, NSObject inheritance, and ObjC CI steps

Testing

• sh -n scripts/generate_docs.sh
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/docs.yml"); puts "ok"'
• git diff --check -- Gemfile scripts/generate_docs.sh
• swift package dump-package
• xcodebuild test -project LetterAvatarKit.xcodeproj -scheme LetterAvatarKit -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.6'
• xcodebuild build -project LetterAvatarKitExample/LetterAvatarKitExample.xcodeproj -scheme LetterAvatarKitExample -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.6'

Note: local docs generation via ./scripts/generate_docs.sh was not completed in this environment before the Bundler workflow update because jazzy was not available globally.