docs(env-and-mode): use Module Augumentation for ImportMetaEnv

[N0tExisting]

Description

As described in #2773 and improved in #4989 you can extend import.meta.env and this PR tries to improve the extension even further, by using module augmentation, which avoids a lot of potential issues.

Additional context

N/A

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[bluwy]

The existing style showed in the docs had worked well for me. Can you elaborate what potential issues this new method helps avoid?

[bluwy]

Closing as the one mentioned in the docs currently works.