Reword the section on in-DOM root templates

[skirtles-code]

This change was motivated by #1864.

The section on using an in-DOM template begins by introducing it as a feature used without a build step. That is one use case, but it is also commonly used with server-side frameworks that generate the template HTML dynamically.

While import { createApp } from 'vue' can be used 'without a build step', it does seem less natural in that context. This was the problem originally raised in #1864. The idea of using import maps is discussed earlier in the docs, but this specific example leans on that knowledge more heavily than the rest of the docs.

Instead, I've started by introducing the feature without explicit motivation, contrasting it with the more common approach. I've then mentioned the use cases at the end, including a link back to the section about using Vue without build tools.

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