docs: add upgrade notes for llm-d version migrations

## Summary

KServe v0.19 upgraded llm-d from v0.6 to v0.7 and v0.20 upgraded to v0.8. While migrations are handled automatically by the controller, the upgrade guide (`docs/install/upgrade-guide.md`) has no mention of these changes or any version-specific upgrade notes for v0.18, v0.19, or v0.20.

## What needs to be documented

### v0.19: llm-d v0.6 → v0.7 migration
- Plugin renames: `prefill-header-handler` → `disagg-headers-handler`, `pd-profile-handler` → `disagg-profile-handler`
- Parameter restructure: `deciderPluginName` → `deciders` map
- Metric flag migration: deprecated CLI flags moved to `core-metrics-extractor` plugin
- Non-zero threshold migration to `prefix-based-pd-decider`
- WVA upgraded to v0.7.0
- Note: all migrations are automatic during reconciliation

### v0.20: llm-d v0.8 and CRD migration
- Routing resources migrated to llm-d.ai CRDs (from GIE experimental CRDs)
- GIE upgraded to v1.5.0
- Envoy AI Gateway upgraded to v1.0.0, Envoy Gateway to v1.8.1
- Gateway API bumped to v1.5.1

### General
- Add version-specific sections to the upgrade guide for v0.18, v0.19, and v0.20

## Related PRs

- https://github.com/kserve/kserve/pull/5433 (llm-d v0.7 migration logic, v0.19)
- https://github.com/kserve/kserve/pull/5560 (non-zero threshold migration, v0.19)
- https://github.com/kserve/kserve/pull/5596 (llm-d v0.8 upgrade, v0.20)
- https://github.com/kserve/kserve/pull/5585 (llm-d.ai CRD migration, v0.20)
- https://github.com/kserve/kserve/pull/5723 (Envoy AI Gateway v1.0.0, v0.20)
- https://github.com/kserve/kserve/pull/5478 (Gateway API v1.5.1, v0.20)

## Since

KServe v0.19, v0.20

Edit docs/install/upgrade-guide.md and review the related migration PRs for the v0.18, v0.19, and v0.20 changes. Add version-specific sections covering the listed llm-d, CRD, gateway, and dependency migrations, including that controller reconciliation handles them automatically. Done means the upgrade guide documents each requested migration and version-specific note.

Written by the indexing model from the issue text.