docs: distinguish the two "env var"s using different words

[sapphi-red]

Description

The "Env Variables and Modes" guide had mixed two "env var"s:

• Env var on the machine: the variables that can be referenced by $FOO in bash, $env:FOO in powershell
• Env var in Vite: build-time constants that are replaced in build, exposed in import.meta.env

This was confusing and led people to understand that "Env vars in Vite" can be used not only for build-time constants.

The way this PR tries to solve is:

• Explain that Vite has a build-time constant feature and env vars are automatically added to that, and organize the sections to be in that order
• Use the word "constants" instead of "env variables" for "Env var in Vite"
  • Maybe it should be kept "env variables" but put "build-time" in front ("build-time env variables") as the frameworks uses "env variables" and would be confusing if we rename it
    • https://svelte.dev/docs/kit/$env-static-public
    • https://docs.astro.build/en/guides/environment-variables/#default-environment-variables
    • https://remix.run/docs/en/main/guides/envvars#browser-environment-variables
    • https://docs.solidjs.com/configuration/environment-variables#environment-variables
    • https://qwik.dev/docs/guides/env-variables/

Preview: https://deploy-preview-18941--vite-docs-main.netlify.app/guide/constants-and-mode.html