Bump @springio/asciidoctor-extensions in /docs

Bumps [@springio/asciidoctor-extensions](https://github.com/spring-io/asciidoctor-extensions) from 1.0.0-alpha.17 to 1.0.0-alpha.18.
- [Changelog](https://github.com/spring-io/asciidoctor-extensions/blob/main/CHANGELOG.adoc)
- [Commits](https://github.com/spring-io/asciidoctor-extensions/compare/v1.0.0-alpha.17...v1.0.0-alpha.18)

---
updated-dependencies:
- dependency-name: "@springio/asciidoctor-extensions"
  dependency-version: 1.0.0-alpha.18
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

docs/package.json

@@ -5,6 +5,6 @@
     "@antora/collector-extension": "1.0.3",
     "@asciidoctor/tabs": "1.0.0-beta.6",
     "@springio/antora-extensions": "1.14.7",
-    "@springio/asciidoctor-extensions": "1.0.0-alpha.17"
+    "@springio/asciidoctor-extensions": "1.0.0-alpha.18"
   }
 }