docs: create a page on built-in fetch

docs/content/1.guide/6.fetch.md

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+---
+icon: i-ri-global-line
+---
+
+# Fetch
+
+Nitro provides a built-in fetch API that can be used to get data from server endpoints or from other sources. It's build on top of the [unjs/ofetch](https://ofetch.unjs.io) package.
+
+This is important to use it instead of the built-in Node `fetch` or Axios because it's optimized for serverless environments and agnostic to the runtime.
+
+::alert{type="info"}
+We **do not overwrite** the built-in `fetch` function, so you can still use it if you want.
+::
+
+## Usage
+
+In you handler, you've just to call the `$fetch` function to make a request. Automatically, the response will be smartly parsed using [unjs/destr](https://destr.unjs.io).
+
+```ts [Router Handler]
+export default defineEventHandler(async (event) => {
+  const data = await $fetch('https://api.github.com/orgs/unjs/repos')
+
+  return data
+})
+```
+
+You can pass a generic type to the `$fetch` function to get a better type inference:
+
+```ts [Router Handler]
+import { Repo } from '~/types'
+
+export default defineEventHandler(async (event) => {
+  const data = await $fetch<Repo[]>('https://api.github.com/orgs/unjs/repos')
+
+  return data
+})
+```
+
+You can pass many options to the `$fetch` function like the method, headers, body, query, etc.
+
+```ts [Router Handler]
+import { Repo } from '~/types'
+
+export default defineEventHandler(async (event) => {
+  const data = await $fetch<Repo[]>('https://api.github.com/orgs/unjs/repos', {
+    method: 'POST',
+    headers: {
+      'Content-Type': 'application/json'
+    },
+    body: {
+      name: 'my-repo'
+    }
+  })
+
+  return data
+})
+```
+
+See more about the usage of the `$fetch` function in the [unjs/ofetch](https://ofetch.unjs.io) documentation.
+
+## Internal Fetch
+
+You can also use the `$fetch` function to make internal requests to other handlers.
+
+```ts [Router Handler]
+export default defineEventHandler(async (event) => {
+  const data = await $fetch('/api/users')
+
+  return data
+})
+```
+
+In reality, no fetch request is made and the handler is directly called thanks to [unjs/unenv](https://unenv.unjs.io). This is useful to avoid making HTTP requests and to slow down your application.