fix gitbook building (#982)

Author: kazupon

docs/en/README.md

@@ -0,0 +1 @@
+{% include "./SUMMARY.md" %}

docs/en/README.md

@@ -0,0 +1 @@
+{% include "./SUMMARY.md" %}

docs/ja/README.md

@@ -0,0 +1 @@
+{% include "./SUMMARY.md" %}