Bump @springio/antora-extensions from 1.12.0 to 1.13.0 in /docs

Bumps [@springio/antora-extensions](https://github.com/spring-io/antora-extensions) from 1.12.0 to 1.13.0.
- [Changelog](https://github.com/spring-io/antora-extensions/blob/main/CHANGELOG.adoc)
- [Commits](spring-io/antora-extensions@v1.12.0...v1.13.0)

---
updated-dependencies:
- dependency-name: "@springio/antora-extensions"
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

docs/package.json

@@ -4,7 +4,7 @@
     "@antora/atlas-extension": "1.0.0-alpha.2",
     "@antora/collector-extension": "1.0.0-alpha.7",
     "@asciidoctor/tabs": "1.0.0-beta.6",
-    "@springio/antora-extensions": "1.12.0",
+    "@springio/antora-extensions": "1.13.0",
     "@springio/asciidoctor-extensions": "1.0.0-alpha.12"
   }
 }