docs: improve "Router to Contract" (#752)

I haven't tested it.  
Only written the result of this Github Discussion
https://github.com/unnoq/orpc/discussions/696

Closes: https://github.com/unnoq/orpc/issues/732

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Documentation**
* Improved guidance on TypeScript typing for imported contract JSON,
including a safer example that preserves type inference and type safety
when using contracts on the client side.
* Added a warning note explaining the necessity of manual casting due to
JSON serialization limitations of standard schema types.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: unnoq <contact@unnoq.com>

Author: 3 people

apps/content/docs/contract-first/router-to-contract.md

@@ -45,7 +45,11 @@ However, if you're deriving the contract from a [router](/docs/router), importin
     ```ts
     import contract from './contract.json' // [!code highlight]
 
-    const link = new OpenAPILink(contract as any, {
+    const link = new OpenAPILink(contract as typeof router, {
       url: 'http://localhost:3000/api',
     })
     ```
+
+    ::: warning
+    Cast `contract` to `typeof router` to ensure type safety, since standard schema types cannot be serialized to JSON so we must manually cast them.
+    :::