feat(learn/userland-migrations): add intro

apps/site/pages/en/learn/migrations/introduction.md

@@ -0,0 +1,27 @@
+---
+title: Userland Migrations
+layout: learn
+authors: JakobJingleheimer
+---
+
+# Userland Migrations
+
+Node.js provide migrations for "userland" code to facilitate adoption of new features and upgrading source-code affected by breaking changes. These are done in collaboration with [`codemod`](https://www.codemod.com), who also work with other major projects like Next.js, React, and Tailwind. Node.js's migrations live in the [`github://nodejs/userland-migrations`](https://github.com/nodejs/userland-migrations) repository and are overseen by the [`@userland-migrations`](https://github.com/orgs/nodejs/teams/userland-migrations) team.
+
+Official migrations are published under the `@nodejs` namespace within the [codemod registry](https://codemod.com/registry?framework=node.js). These have been reviewed and/or authored by Node.js collaborators. There are also unofficial migrations available which have not been reviewed by Node.js.
+
+A migration alters a project's source-code to apply a new design pattern, like:
+
+```console
+$ cd path/to/your/project
+$ npx codemod@latest @nodejs/correct-ts-specifiers
+```
+
+The cited migration transforms legacy typescript imports to standards-compliant specifiers like:
+
+```ts displayName="before"
+import Foo from './foo';
+```
+```ts displayName="after"
+import type Foo from './foo/index.ts';
+```