add multi-path dependency conflict guide; update changelog with stability/compat notes

[Kexin2000]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

• Adds a new user guide “Handling Multi-Path Dependency Conflicts and Best Practices”:
  • Explains policy aggregation rules, explicit-policy precedence, event meanings, troubleshooting, and operational best practices.
  • Path: docs/guides/dependency-multi-path-best-practices.md
• Updates CHANGELOG to note more stable dependency handling, enhanced observability, no CRD changes, and backward compatibility.
  • Path: docs/CHANGELOG/CHANGELOG-1.15.md

Which issue(s) this PR fixes:
Fixes #6000

Special notes for your reviewer:

• Documentation-only changes; no behavior or API changes.

Does this PR introduce a user-facing change?:

Docs: Add guide “Handling Multi-Path Dependency Conflicts and Best Practices”, covering aggregation rules, explicit precedence, events, troubleshooting, and best practices. Update CHANGELOG with stability and compatibility notes; no CRD change.

[karmada-bot]

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