refactor(docs-infra): scope select-menu styles to the <aio-select> element (#40704)

This commit ensures that all styles for the select-menu element (which
are defined in `_select-menu.scss`) only apply to elements inside an
`<aio-select>` element. This will prevent the styles accidentally taking
effect on a different part of the app.

PR Close #40704

aio/src/styles/2-modules/_select-menu.scss

@@ -1,5 +1,6 @@
 /* SELECT MENU */
 
+aio-select {
   .form-select-menu {
     position: relative;
   }
@@ -87,3 +88,4 @@
       }
     }
   }
+}