docs: migrate UI docs (#13245)

* docs: create a new UI docs project (#13233)

* docs: create a new UI docs project

* fix installation errors

* docs: migrate UI docs content to new project (#13241)

* Fix content

* added examples for some components

* finish adding examples

* lint fix

* fix build errors

* delete empty files

* path fixes + refactor

* fix build error

www/apps/ui/specs/examples/heading-demo.tsx

@@ -0,0 +1,11 @@
+import { Heading } from "@medusajs/ui"
+
+export default function HeadingDemo() {
+  return (
+    <div className="flex flex-col items-center">
+      <Heading level="h1">This is an H1 heading</Heading>
+      <Heading level="h2">This is an H2 heading</Heading>
+      <Heading level="h3">This is an H3 heading</Heading>
+    </div>
+  )
+}