docs(mobile/3): note Inertia 3 axios requirement in installation guide

NativePHP Mobile intercepts axios at bundle time to route HTTP requests
through the embedded PHP runtime. Inertia 3 removed axios in favour of
native fetch, so fresh Inertia 3 projects fail the NativePHP build until
they reinstall axios and register the Inertia axios adapter.

Adds an aside to mobile/3/getting-started/installation explaining the
requirement, the fix (`npm install axios @inertiajs/axios-adapter`), and
links to the Inertia client-side-setup docs.

Pairs with NativePHP/mobile-air#100 which makes the Vite plugin fail
early with an error pointing here and at the Inertia adapter docs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: shanerbaner82

resources/views/docs/mobile/3/getting-started/installation.md

@@ -92,6 +92,27 @@ the heavy lifting for you.
 
 For this reason, we also recommend you add the `nativephp` folder to your `.gitignore`.
 
+<aside>
+
+#### Using Inertia 3?
+
+NativePHP Mobile routes every outgoing HTTP request through the embedded PHP runtime
+by intercepting `axios` at bundle time. [Inertia 3 removed axios][inertia-axios] in
+favour of native `fetch`, so fresh Inertia 3 projects won't have it installed — the
+NativePHP build fails until you add it back and wire up the Inertia axios adapter:
+
+```shell
+npm install axios @inertiajs/axios-adapter
+```
+
+Then follow the [Inertia docs on using axios][inertia-axios] to register the adapter
+in your client-side setup. Apps on Inertia 2 (or using axios directly) already have
+it installed and don't need any change.
+
+[inertia-axios]: https://inertiajs.com/docs/v3/installation/client-side-setup#using-axios
+
+</aside>
+
 ## Start your app
 
 **Heads up!** Before starting your app in a native context, try running it in the browser. You may bump into exceptions