docs: document custom serialization adapters in Start #7796

The Router SSR guide still says application-level serialization customization is being worked on. Start now supports serializationAdapters through createStart, and applies them to SSR hydration and server-function serialization.

Update the stale SSR guidance and add Start documentation that covers:

• Creating an adapter with createSerializationAdapter
• Registering adapters through createStart({ serializationAdapters })
• A custom type example such as Prisma Decimal
• Adapter use in SSR and server-function payloads
• Built-in serialization support, including BigInt

Related Work: #3785