docs: first pass at the shared directory documentation

docs/2.guide/2.directory-structure/1.shared.md

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+---
+title: 'shared'
+head.title: 'shared/'
+description: Use the shared/ directory to share functionality between the Vue app and the Nitro server.
+navigation.icon: i-ph-folder
+---
+
+The `shared/` directory allows you to share code that can be used in both the Vue app and the Nitro server.
+
+::tip
+The `shared/` directory is available in Nuxt v3.14+.
+::
+
+::important
+Code in the shared directory cannot import any Vue or nitro code.
+::
+
+## Usage
+
+**Method 1:** Using named export
+
+```ts twoslash [shared/utils/capitalize.ts]
+export const capitalize = (input: string) => {
+  return input[0].toUpperCase() + input.slice(1)
+}
+```
+
+**Method 2:** Using default export
+
+```ts twoslash [shared/utils/capitalize.ts]
+export default function capitalize (input: string) {
+  return input[0].toUpperCase() + input.slice(1)
+}
+```
+
+**Usage:** You can now use auto-imported utility functions in `.js`, `.ts` and `.vue` files within your Vue app and the `server/` directory. 
+
+If you are using `compatibilityVersion: 4`, you can use the auto-imported functions in the `app/` directory.
+
+```vue [app.vue]
+<script setup lang="ts">
+const hello = capitalize('hello')
+</script>
+
+<template>
+  <div>
+    {{ hello }}
+  </div>
+</template>
+```
+
+```ts [server/api/hello.get.ts]
+export default defineEventHandler((event) => {
+  return {
+    hello: capitalize('hello')
+  }
+})
+```
+
+## Auto Imports
+
+Only files in the `shared/utils/` and `shared/types/` directories will be auto-imported.
+
+```bash [Directory Structure]
+-| shared/
+---| foo.ts        # Not auto-imported
+---| utils/
+-----| bar.ts      # Auto-imported
+---| types/
+-----| bar.d.ts    # Auto-imported
+```
+
+Any other files you create in the shared folder must be manually imported using `import foo from '#shared/foo'`.
+
+This includes nested directories. A file located at `shared/formatters/lower.ts` will be imported with `import lower from '#shared/formatters/lower'`.
+
+:read-more{to="/docs/guide/concepts/auto-imports"}