Docs: Explain the entry.client.{jsx,tsx} file (#8987)

RW v6 (our implementation of Vite support really) introduces a new file,
`entry.client.{jsx,tsx}`.
This PR adds a short description of this file to the tutorial

This is a follow-up to the changes made here
#8959

---------

Co-authored-by: Dominic Saadi <[email protected]>

docs/docs/tutorial/chapter1/file-structure.md

@@ -142,8 +142,9 @@ That's it for the backend.
     - `NotFoundPage.{jsx,tsx}` will be served when no other route is found (see `Routes.{jsx,tsx}` below).
     - `FatalErrorPage.{jsx,tsx}` will be rendered when there is an uncaught error that can't be recovered from and would otherwise cause our application to really blow up (normally rendering a blank page).
   - `App.{jsx,tsx}` the bootstrapping code to get our Redwood app up and running.
+  - `entry.client.{jsx,tsx}` is the standard React starting point for our app.
   - `index.css` is a good starting place for custom CSS, but there are many options (we like [TailwindCSS](https://tailwindcss.com/) which, believe it or not, may not require you to write any custom CSS for the life of your app!)
-  - `index.html` is the standard React starting point for our app.
+  - `index.html` is what's first sent to a visitor's browser. It fetches `entry.client.{jsx,tsx}`.
   - `Routes.{jsx,tsx}` the route definitions for our app which map a URL to a _Page_.
 
 We'll dip in and out of these directories and files (and create some new ones) as we work through the tutorial.