Enabling linkattrs explicitly on doc pages having external links

Asciidoc may fail to render link attributes for external links
supposed to open in a new window correctly. This change adds
:linkattrs: to the beginning of such files to force parsing link
attributes correctly.

Bug: Issue 12068
Change-Id: If18be60de646ff78f672239dd4fa435fd4fd92ab
diff --git a/Documentation/index.txt b/Documentation/index.txt
index 09cb21f..d270f97 100644
--- a/Documentation/index.txt
+++ b/Documentation/index.txt
@@ -1,3 +1,4 @@
+:linkattrs:
 = Gerrit Code Review for Git
 
 == Quickstarts