fix(docs): update SSR boot file template variable syntax for app-vite v2 (quasarframework#18247)

In @quasar/app-vite v2, the render-template is compiled with
{ variable: 'ssrContext' }, which scopes all interpolations under
the ssrContext object. The previously documented {{ someProp }} syntax
no longer works; the correct syntax is {{ ssrContext.someProp }}.
Updated the example in the Boot Files section of the Configuring SSR
page to reflect this change, and added a tip callout to help users
upgrading from v1.

Author: Jadu07

docs/src/pages/quasar-cli-vite/developing-ssr/configuring-ssr.md

@@ -236,12 +236,16 @@ When a boot file runs on the server, you will have access to one more parameter
 export default ({ app, ..., ssrContext }) => {
   // You can add props to the ssrContext then use them in the /index.html.
   // Example - let's say we ssrContext.someProp = 'some value', then in index template we can reference it:
-  // {{ someProp }}
+  // {{ ssrContext.someProp }}
 }
 ```
 
-When you add such references (`someProp` surrounded by brackets in the example above) into your `/index.html`, make sure you tell Quasar it’s only valid for SSR builds:
+When you add such references into your `/index.html`, make sure you tell Quasar it's only valid for SSR builds:
+
+::: tip
+As of `@quasar/app-vite` v2, template variables in `/index.html` must be scoped under the `ssrContext` object. Use `{{ ssrContext.someProp }}` instead of the v1 form `{{ someProp }}`.
+:::
 
 ```html /index.html
-<% if (ctx.mode.ssr) { %>{{ someProp }} <% } %>
+<% if (ctx.mode.ssr) { %>{{ ssrContext.someProp }} <% } %>
 ```