chore(docs): add section for Custom Type Declarations (#60282)

We used to have docs but it was lost in the rewrite (as seen in this
comment
#30060 (comment))

This PR adds back the documentation for Custom Type Declarations.

Closes NEXT-1999

docs/02-app/01-building-your-application/07-configuring/01-typescript.mdx

@@ -294,6 +294,27 @@ module.exports = {
 }
 ```
 
+## Custom Type Declarations
+
+When you need to declare custom types, you might be tempted to modify `next-env.d.ts`. However, this file is automatically generated, so any changes you make will be overwritten. Instead, you should create a new file, let's call it `new-types.d.ts`, and reference it in your `tsconfig.json`:
+
+```json filename="tsconfig.json"
+{
+  "compilerOptions": {
+    "skipLibCheck": true
+    //...truncated...
+  },
+  "include": [
+    "new-types.d.ts",
+    "next-env.d.ts",
+    ".next/types/**/*.ts",
+    "**/*.ts",
+    "**/*.tsx"
+  ],
+  "exclude": ["node_modules"]
+}
+```
+
 ## Version Changes
 
 | Version   | Changes                                                                                                                              |