Dedicated H3 to accessing Env Vars

[sebscholl]

Description

I propose dedicating a h3 section to "Accessing Environment Variables in Application Code" that more clearly calls out how env vars need to be prefixed with VUE_ to be accessed via import.meta.env in client source code.

Additional context

I felt that this information was hidden within the .env Files section when it deserves its own nested section.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[bluwy]

In #8989 I added a code example showing how to access them, which I think should catch the attention for using it. I'll close this in favour of it.