docs: add validation hook details for webpack 5.106 (#8158)

Author: bjohansebas

src/content/api/compiler-hooks.mdx

@@ -89,6 +89,27 @@ Triggered after resolver setup is complete.
 
 - Callback Parameters: `compiler`
 
+### validate
+
+`SyncHook`
+
+<Badge text="5.106.0+" />
+
+Called to register validation work for webpack configuration, plugins, and loaders. Plugin authors can tap this hook and use `compiler.validate(...)` to participate in webpack's built-in validation flow.
+
+W> The hook is always available. `compiler.validate(...)` skips validation when `validate: false` is set. With `experiments.futureDefaults`, webpack keeps validation enabled by default in development and disables it by default in production.
+
+```js
+compiler.hooks.validate.tap("MyPlugin", () => {
+  compiler.validate(
+    () => require("./schema/MyPlugin.json"),
+    options,
+    { name: "My Plugin", baseDataPath: "options" },
+    (validatedOptions) => require("./schema/MyPlugin.check")(validatedOptions),
+  );
+});
+```
+
 ### initialize
 
 `SyncHook`

src/content/contribute/writing-a-plugin.mdx

@@ -89,7 +89,7 @@ export default {
 };
 ```
 
-Use [`schema-utils`](https://github.com/webpack/schema-utils) in order to validate the options being passed through the plugin options. Here is an example:
+A familiar way to validate plugin options in webpack versions **before `5.106`** is to use [`schema-utils`](https://github.com/webpack/schema-utils) in the constructor:
 
 ```js
 import { validate } from "schema-utils";
@@ -116,6 +116,27 @@ export default class HelloWorldPlugin {
 }
 ```
 
+Starting in webpack `5.106`, you can move that validation into `apply()` by tapping `compiler.hooks.validate` and calling `compiler.validate(...)` instead:
+
+```js
+export default class HelloWorldPlugin {
+  constructor(options = {}) {
+    this.options = options;
+  }
+
+  apply(compiler) {
+    compiler.hooks.validate.tap("HelloWorldPlugin", () => {
+      compiler.validate(
+        () => require("./schema/hello-world-plugin.json"),
+        this.options,
+      );
+    });
+  }
+}
+```
+
+W> This validation flow is skipped when webpack is configured with `validate: false`. With `experiments.futureDefaults`, validation is enabled by default in development mode and disabled by default in production mode.
+
 ## Compiler and Compilation
 
 Among the two most important resources while developing plugins are the [`compiler`](/api/node/#compiler-instance) and [`compilation`](/api/compilation-hooks/) objects. Understanding their roles is an important first step in extending the webpack engine.