docs: add comprehensive migration guide from robfig/cron

[CybotTM]

Summary

• Create docs/MIGRATION.md with comprehensive migration documentation
• Update README.md to link to the new migration guide with a tip callout

Migration Guide Contents

The guide covers:

Quick Start

• Import path changes
• go.mod updates
• Go version requirements (1.13 → 1.25)

Behavioral Differences

Detailed before/after examples for:

• TZ= parsing panic fixes (cron.ParseStandard("TZ=0") causes panic robfig/cron#554, Fix panic when missing optional fields or descriptor after timezone robfig/cron#555)
• Entry.Run() chain behavior fix (SkipIfStillRunning Chain Not Applied When Manually Calling Entry.Job.Run() robfig/cron#551)
• DST spring-forward handling (Don't skip crons when time jumps forward due to DST robfig/cron#541)
• Step range validation (ParseStandard not validating step range robfig/cron#543)
• Minimum @every duration enforcement
• Input length limits (1024 chars)

Type Changes

• EntryID: int → uint64 with migration examples

New Features

• FakeClock for deterministic testing
• StopAndWait() convenience method
• Timeout wrapper (with caveats)
• Heap-based scheduling performance improvements
• slog adapter

Practical Guidance

• Migration checklist
• Testing recommendations with code examples
• Troubleshooting section for common issues

Test Plan

• Markdown renders correctly (verified locally)
• All internal links work
• Code examples are syntactically correct
• README tip callout renders properly
• Tests pass (go test -race ./...)
• Linter passes (golangci-lint run ./...)

Closes #73