fix links problem

Author: mxz96102

_config.yml

@@ -27,6 +27,7 @@ defaults:
     values:
       layout: "doc"
       nav_exclude: true
+      prefix: '/sqlflow/'
       grand_parent: Documents
   - scope:
       path: "sqlflow/doc/installation.md"

_layouts/doc.md

@@ -10,6 +10,8 @@ layout: default
 a {color: #1BA2FF}
 </style>
 
-{{content | replace: "README.md", "" | replace: ".md", "" | replace: '/example/', '../example/' }}
+{% assign href_prefix = 'href="' | append: page.prefix %}
+
+{{content | replace: "README.md", "" | replace: ".md", "" | replace: 'href="/', href_prefix }}