Docs: custom Fastly entry point section for runtime_env_config / dispatch_with_registries #361

Follow-up to #351, which exported runtime_env_config and request::dispatch_with_registries for custom Fastly entry points.

docs/guide/blob-app-config-migration.md (~236-260) already documents the edgezero_runtime_env store, the local-server block, and the missing-store warning text, so it is the natural place for a short custom-entry-points subsection: build explicit StoresMetadata (the default Hooks::stores() is empty), call runtime_env_config, dispatch via dispatch_with_registries. The motivating consumer is external (Trusted Server), so discoverability is load-bearing for the fix.