Add v1.x info banner on every v1 docs page

Docusaurus's built-in DocVersionBanner only renders when a version
declares a `banner` in docusaurus.config.js. v2.0 already ships an
"unreleased" warning banner, but v1.x carried no banner — readers had
to consult the version switcher in the navbar to know which major
version they were viewing.

Swizzle DocVersionBanner so v1.x pages render an info callout that
states they are on v1.x (current stable) and links to the v2.0
introduction for preview. v2.0 pages continue to render the original
unreleased banner unchanged. Addresses the "v1/v2 visual distinction
on every page" deliverable in Docs Phase 0 (issue #234).

Verified via npm run build that the banner appears on v1 pages and
not on v2 pages, and that no internal links are broken.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: durable-workflow-ops

src/theme/DocVersionBanner/index.js

@@ -0,0 +1,33 @@
+import React from 'react';
+import DocVersionBanner from '@theme-original/DocVersionBanner';
+import Link from '@docusaurus/Link';
+import {useDocsVersion} from '@docusaurus/theme-common/internal';
+
+// Docusaurus only renders the built-in banner when a version declares
+// `banner` in docusaurus.config.js. v2.0 already shows an "unreleased"
+// warning banner; v1.x carries no banner by default. Until v2.0 ships,
+// this wrapper renders an info banner on v1.x pages so every page gives
+// readers a visual cue about which major version they are viewing —
+// not only the version switcher in the navbar.
+export default function DocVersionBannerWrapper(props) {
+  const versionMetadata = useDocsVersion();
+
+  if (versionMetadata.version === '1.x') {
+    return (
+      <div
+        className="alert alert--info margin-bottom--md"
+        role="alert">
+        <div>
+          You are viewing the <b>v1.x</b> documentation (the current stable
+          release line).
+        </div>
+        <div className="margin-top--md">
+          v2.0 is in active development —{' '}
+          <Link to="/docs/2.0/introduction">preview the v2.0 documentation</Link>.
+        </div>
+      </div>
+    );
+  }
+
+  return <DocVersionBanner {...props} />;
+}