fix(manifests): add docs/de-DE to package.json publish surface

The npm-publish-surface test asserts package.json `files` aligns with
the install module graph. The new docs-de-de module requires docs/de-DE
in the published files list.

Refs #1980

package.json

@@ -56,6 +56,7 @@
     "agent.yaml",
     "agents/",
     "commands/",
+    "docs/de-DE/",
     "docs/ja-JP/",
     "docs/ko-KR/",
     "docs/pt-BR/",