docs: add unreleased banner to v2.0 documentation pages

Add banner: "unreleased" to version config for current (2.0) version.

Result: All v2.0 docs pages at /docs/2.0/ now show a yellow warning banner:
"This is unreleased documentation for Durable Workflow 2.0 version.
For up-to-date documentation, see the latest version (1.x)."

Improves user clarity for prerelease documentation. Users landing on v2 pages
via search or direct links now have clear indication they are viewing
unreleased docs with a link back to stable 1.x.

Fixes #259

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: durable-workflow-ops

docusaurus.config.js

@@ -42,6 +42,7 @@ const config = {
             current: {
               label: '2.0',
               path: '2.0',
+              banner: 'unreleased',
             },
             '1.x': {
               label: '1.x',