Update index.mdx

Removed at its crux because this phrase confuses the users who are not native speakers. The phrase "at its crux" means "at its core" or "at its essence." It doesn't provide the useful information for this sentence, so can be removed.

website/docs/guides/docs/sidebar/index.mdx

@@ -64,7 +64,7 @@ You can also define your sidebars explicitly.
 
 ## Sidebar object {#sidebar-object}
 
-A sidebar at its crux is a hierarchy of categories, doc links, and other hyperlinks.
+A sidebar is a hierarchy of categories, doc links, and other hyperlinks.
 
 ```ts
 type Sidebar =