Organize SCSS: split implicit general styling and explicit. used classes

So differentiate between general stuff like variables, implicit general
document styling and classes, which need to be used explicitly.

themes/ccchh/assets/scss/main.scss

@@ -1,3 +1,8 @@
+// General variables.
 @import "variables.scss";
-@import "container.scss";
+
-@import "document-typography.scss";
+// General implicit document styling.
+@import "document/typography.scss";
+
+// Classes to be used explicitly.
+@import "classes/container.scss";