feat: pass loaded siteConfig as props for React component

lib/load/config.js

@@ -35,6 +35,17 @@ module.exports = function loadConfig(siteDir, deleteCache = true) {
     );
   }
 
+  /* Fill default value */
+  const defaultConfig = {
+    customDocsPath: 'docs',
+    docsUrl: 'docs'
+  };
+  Object.keys(defaultConfig).forEach(field => {
+    if (!config[field]) {
+      config[field] = defaultConfig[field];
+    }
+  });
+
   /* 
     User's own array of custom fields, 
     e.g: if they want to include some field so they can access it later from `props.siteConfig`