feat: refresh exercise and update to latest guidelines

[chriswblake]

This pull request refactors and simplifies the "Resolve Merge Conflicts" exercise by restructuring its steps, improving workflow logic, and updating the content for clarity and consistency. The changes involve replacing older markdown files and workflows with new, streamlined versions, as well as updating the automation logic to use more modern tools and conventions.

Content Updates:

• Replaced outdated step files (1-create-a-pr.md, 2-resolve-a-merge-conflict.md, 3-create-your-own-conflict.md, 4-merge-your-pull-request.md, X-finish.md) with new, concise versions (1-step.md, 2-step.md, 3-step.md, x-review.md) that provide clearer instructions and improved formatting. [1] [2] [3] [4] [5] [6] [7] [8]

Workflow Improvements:

• Updated .github/workflows/0-start-exercise.yml to use version v0.7.0 of the skills/exercise-toolkit, replaced manual GitHub CLI commands with reusable actions (e.g., GrantBirki/comment@v2.1.1), and streamlined environment variable usage. [1] [2] [3] [4]
• Removed redundant workflows (1-create-a-pr.yml, 2-resolve-a-merge-conflict.yml) and replaced them with simplified versions (1-step.yml). These new workflows use updated logic and tools for better maintainability. [1] [2] [3]

Automation Enhancements:

• Improved automation for posting step content and progress updates by leveraging reusable GitHub Actions instead of custom scripts, reducing complexity and potential errors. [1] [2]

These changes collectively enhance the exercise's usability, maintainability, and clarity, making it easier for users to learn how to resolve merge conflicts effectively.