Bump @antora/atlas-extension in /docs (#1037)

Bumps [@antora/atlas-extension](https://gitlab.com/antora/antora-atlas-extension) from 1.0.0-alpha.5 to 1.0.0-alpha.6.
- [Changelog](https://gitlab.com/antora/antora-atlas-extension/blob/main/CHANGELOG.adoc)
- [Commits](https://gitlab.com/antora/antora-atlas-extension/compare/v1.0.0-alpha.5...v1.0.0-alpha.6)

---
updated-dependencies:
- dependency-name: "@antora/atlas-extension"
  dependency-version: 1.0.0-alpha.6
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

docs/package.json

@@ -1,7 +1,7 @@
 {
   "dependencies": {
     "antora": "3.2.1",
-    "@antora/atlas-extension": "1.0.0-alpha.5",
+    "@antora/atlas-extension": "1.0.0-alpha.6",
     "@antora/collector-extension": "1.0.3",
     "@asciidoctor/tabs": "1.0.0-beta.6",
     "@springio/antora-extensions": "1.14.13",