Add basePath documentation (#14882)

Fixes #14453

Author: timneutkens

docs/api-reference/next.config.js/basepath.md

@@ -0,0 +1,43 @@
+---
+description: Learn more about setting a base path in Next.js
+---
+
+# Base Path
+
+To deploy a Next.js application under a sub-path of a domain you can use the `basePath` option.
+
+`basePath` allows you to set a path prefix for the application. For example `/docs` instead of `/` (the default).
+
+For example, to set the base path to `/docs`, set the following configuration in `next.config.js`:
+
+```js
+module.exports = {
+  basePath: '/docs',
+}
+```
+
+## Links
+
+When linking to other pages using `next/link` and `next/router` the `basePath` will be automatically applied.
+
+For example using `/about` will automatically become `/docs/about` when `basePath` is set to `/docs`.
+
+```js
+export default function HomePage() {
+  return (
+    <>
+      <Link href="/about">
+        <a>About Page</a>
+      </Link>
+    </>
+  )
+}
+```
+
+Output html:
+
+```html
+<a href="/docs/about">About Page</a>
+```
+
+This makes sure that you don't have to change all links in your application when changing the `basePath` value.

docs/manifest.json

@@ -227,6 +227,10 @@
               "title": "Environment Variables",
               "path": "/docs/api-reference/next.config.js/environment-variables.md"
             },
+            {
+              "title": "Base Path",
+              "path": "/docs/api-reference/next.config.js/basepath.md"
+            },
             {
               "title": "Custom Page Extensions",
               "path": "/docs/api-reference/next.config.js/custom-page-extensions.md"