Add environment variable documentation for HTML and Static assets

[rdhelms]

In Vue 2 documentation, there was a very helpful section about HTML and Static Assets, which specifically provided guidance on using interpolated values in the entry index.html file. The lack of this similar documentation for Vue 3 makes it more confusing to migrate Vue 2 projects that used that pattern.

https://cli.vuejs.org/guide/html-and-static-assets.html#the-index-file

[rdhelms]

Realized I accidentally created this here instead of in the vite docs - closing this in favor of vitejs/vite#12579