Update font links and add missing error (#41910)

Adds an error that was missing. Link to beta until the changes are live.

closes #41900

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the
feature request has been accepted for implementation before opening a
PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing
doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

errors/google-fonts-missing-subsets.md

@@ -0,0 +1,34 @@
+# Missing specified subset for a `@next/font/google` font
+
+#### Why This Error Occurred
+
+Preload is enabled for a font that is missing a specified subset.
+
+#### Possible Ways to Fix It
+
+Specify which subsets to preload for that font.
+
+- On a font per font basis by adding it to the function call
+
+```js
+const inter = Inter({ subsets: ['latin'] })
+```
+
+- Globally for all your fonts
+
+```js
+// next.config.js
+module.exports = {
+  experimental: {
+    fontLoaders: [
+      { loader: '@next/font/google', options: { subsets: ['latin'] } },
+    ],
+  },
+}
+```
+
+If both are configured, the subset in the function call is used.
+
+### Useful Links
+
+[Specifying a subset](https://beta.nextjs.org/docs/optimizing/fonts#specifying-a-subset)

errors/manifest.json

@@ -745,6 +745,10 @@
         {
           "title": "babel-font-loader-conflict",
           "path": "/errors/babel-font-loader-conflict.md"
+        },
+        {
+          "title": "google-fonts-missing-subsets",
+          "path": "/errors/google-fonts-missing-subsets.md"
         }
       ]
     }

packages/font/README.md

@@ -4,4 +4,4 @@
 
 This new font system also allows you to conveniently use all Google Fonts with performance and privacy in mind. CSS and font files are downloaded at build time and self-hosted with the rest of your static assets. No requests are sent to Google by the browser.
 
-[Read more](https://nextjs.org/docs/optimizing/fonts)
+[Read more](https://beta.nextjs.org/docs/optimizing/fonts)

packages/font/src/google/loader.ts

@@ -46,7 +46,7 @@ const downloadGoogleFonts: FontLoader = async ({
         fontFamily
       )} has no selected subsets. Please specify subsets in the function call or in your ${chalk.bold(
         'next.config.js'
-      )}, otherwise no fonts will be preloaded. Read more: https://nextjs.org/docs/api-reference/components/font#nextfontgoogle`
+      )}, otherwise no fonts will be preloaded. Read more: https://nextjs.org/docs/messages/google-fonts-missing-subsets`
     )
   }