[ja] Change footer l10n from Home to Documentation

Addresses Japanese l10n strings described in #43068

Author: lyzlisa

content/ja/docs/home/_index.md

@@ -5,7 +5,7 @@ noedit: true
 cid: docsHome
 layout: docsportal_home
 class: gridPage gridPageHome
-linkTitle: "ホーム"
+linkTitle: "ドキュメント"
 main_menu: true
 weight: 10
 hide_feedback: true