Update AMP validation docs (#13525)

Updates docs for AMP validation with usage examples for custom validators added in #9191 and `skipValidation` mode added in #10705

docs/advanced-features/amp-support/amp-validation.md

@@ -7,3 +7,27 @@ description: AMP pages are automatically validated by Next.js during development
 AMP pages are automatically validated with [amphtml-validator](https://www.npmjs.com/package/amphtml-validator) during development. Errors and warnings will appear in the terminal where you started Next.js.
 
 Pages are also validated during [Static HTML export](/docs/advanced-features/static-html-export.md) and any warnings / errors will be printed to the terminal. Any AMP errors will cause the export to exit with status code `1` because the export is not valid AMP.
+
+### Custom Validators
+
+You can set up custom AMP validator in `next.config.js` as shown below:
+
+```jsx
+module.exports = {
+  amp: {
+    validator: './custom_validator.js',
+  },
+}
+```
+
+### Skip AMP Validation
+
+To turn off AMP validation add the following code to `next.config.js`
+
+```jsx
+experimental: {
+  amp: {
+    skipValidation: true
+  }
+}
+```