docs: create CHANGELOG.md following Keep a Changelog format

[chernistry]

Summary

Create a unified CHANGELOG.md that compiles all release notes from v1.0 through the current version, following the Keep a Changelog (https://keepachangelog.com) format. This gives users and contributors a single place to see what changed across versions.

Why This Matters

• Standard open-source practice -- users expect a CHANGELOG
• GitHub Releases are great but scattered -- CHANGELOG is one file, one history
• Contributors need to know what changed to avoid duplicate work
• Package managers and distribution tools often look for CHANGELOG.md
• Demonstrates project maturity and professionalism

Format

Follow Keep a Changelog v1.1.0 (https://keepachangelog.com/en/1.1.0/):

# Changelog

All notable changes to this project will be documented in this file.

The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).

## [Unreleased]

### Added
- ...

## [1.3.0] - 2026-XX-XX

### Added
- ...

### Changed
- ...

### Fixed
- ...

## [1.2.0] - 2026-XX-XX
...

## [1.0.0] - 2026-XX-XX

### Added
- Initial release
- English <-> Russian layout detection
- ...

[Unreleased]: https://github.com/chernistry/RightLayout/compare/v1.3.0...HEAD
[1.3.0]: https://github.com/chernistry/RightLayout/compare/v1.2.0...v1.3.0

Categories to Use

• Added -- new features
• Changed -- changes in existing functionality
• Deprecated -- soon-to-be removed features
• Removed -- removed features
• Fixed -- bug fixes
• Security -- vulnerability fixes

Source Material

Compile from:

• GitHub Releases: gh release list --repo chernistry/RightLayout
• Git log: git log --oneline v1.0.0..HEAD
• Release notes directory (if it exists)
• Commit messages for context

Good First Issue

This is marked as a good first issue because:

• No code changes required
• Clear, well-defined output format
• Can be done by reading existing release notes and git history
• Teaches contributor about the project history

Acceptance Criteria

• CHANGELOG.md created in repository root
• All versions from v1.0 to current are documented
• Follows Keep a Changelog format exactly
• Each entry categorized correctly (Added/Changed/Fixed/etc.)
• Comparison links at bottom are correct
• Dates match actual release dates
• [Unreleased] section exists for ongoing work