docs(v3): rewrite upgrade guide for InertiaCore v3.0

Replace the upstream Inertia.js v3.0 upgrade guide with an
InertiaCore-specific rewrite covering the actual C# / ASP.NET Core
changes in this release: unified PropsResolver pipeline, custom
Inertia.HandleExceptionsUsing, per-request SSR opt-out, structured
IGateway.RenderFailed events, and related adapter-level updates.

Author: adrum