chore: deprecate endpoints (#3022)

apps/docs/src/components/mdx/envelope-warning.tsx

@@ -0,0 +1,19 @@
+import { Callout } from 'fumadocs-ui/components/callout';
+
+const MIGRATION_GUIDE_HREF = '/docs/developers/api/migrate-to-envelopes';
+
+/**
+ * Deprecation banner steering API consumers away from the legacy document and
+ * template create endpoints and towards the unified Envelope API.
+ *
+ * Registered globally in `mdx-components.tsx`, so it can be used in any MDX page
+ * as `<EnvelopeWarning />` without an explicit import.
+ */
+export function EnvelopeWarning() {
+  return (
+    <Callout type="error">
+      <strong>Documents and templates are being deprecated and replaced by envelopes.</strong>{' '}
+      <a href={MIGRATION_GUIDE_HREF}>Read the migration guide here.</a>
+    </Callout>
+  );
+}