Add content for "rerere"

[braboj]

As a developer dealing with recurring merge conflicts, I want to understand git rerere so that I can let Git automatically resolve conflicts it has seen before.

What

Add content covering git rerere (reuse recorded resolution) — how it records conflict resolutions and replays them automatically on future occurrences.

Why

Rerere is underused but powerful for long-lived branches and repeated rebases. It fits naturally in the expert topics or playbook chapter.

Acceptance criteria

• Explain what rerere does (records and replays conflict resolutions)
• Show how to enable it (rerere.enabled = true)
• Walk through a practical example (conflict → resolve → rerere replays on next occurrence)
• Cover git rerere forget for clearing bad resolutions
• Mention use cases (long-lived feature branches, repeated rebases)