Replace old docs with new docs repo (#24201)

docs/guides/runtime/set-env.mdx

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+---
+title: Set environment variables
+sidebarTitle: Set env variables
+mode: center
+---
+
+The current environment variables can be accessed via `process.env` or `Bun.env`.
+
+```ts index.ts icon="/icons/typescript.svg"
+Bun.env.API_TOKEN; // => "secret"
+process.env.API_TOKEN; // => "secret"
+```
+
+---
+
+Set these variables in a `.env` file.
+
+Bun reads the following files automatically (listed in order of increasing precedence).
+
+- `.env`
+- `.env.production`, `.env.development`, `.env.test` (depending on value of `NODE_ENV`)
+- `.env.local` (not loaded when `NODE_ENV=test`)
+
+```txt .env icon="settings"
+FOO=hello
+BAR=world
+```
+
+---
+
+Variables can also be set via the command line.
+
+<CodeGroup>
+
+```sh Linux/macOS icon="terminal"
+FOO=helloworld bun run dev
+```
+
+```sh Windows icon="windows"
+# Using CMD
+set FOO=helloworld && bun run dev
+
+# Using PowerShell
+$env:FOO="helloworld"; bun run dev
+```
+
+</CodeGroup>
+
+---
+
+See [Docs > Runtime > Environment variables](https://bun.com/docs/runtime/env) for more information on using environment variables with Bun.