Drop "_s2-docs-" prefix from Sass partials.

docs/css/s2-docs.scss

@@ -1,16 +1,16 @@
 ---
 ---
-@import "s2-result-repository";
+@import "result-repository";
-@import "s2-docs-sidenav";
+@import "sidenav";
-@import "s2-docs-hamburger";
+@import "hamburger";
-@import "s2-docs-layout";
+@import "layout";
-@import "s2-docs-nav";
+@import "nav";
-@import "s2-docs-footer";
+@import "footer";
-@import "s2-docs-alert";
+@import "alert";
-@import "s2-docs-home";
+@import "home";
-@import "s2-docs-examples";
+@import "examples";
-@import "s2-docs-social";
+@import "social";
-@import "s2-docs-buttons";
+@import "buttons";
 @import "anchorjs";
 @import "jumbotron";
 @import "prettify";