fix: workaround Hugo Modules incompatibility with Bootstrap

"In Go Modules, the /vendor directory in a module has a special meaning, so they delete it from the bundle"

Enables Academic to be installed as a Hugo Module.

See https://github.com/gohugoio/hugo/issues/6945

assets/scss/_vendor/bootstrap/_mixins.scss

@@ -3,7 +3,7 @@
 // Used in conjunction with global variables to enable certain theme features.
 
 // Vendor
-@import "vendor/rfs";
+@import "_vendor/rfs";
 
 // Deprecate
 @import "mixins/deprecate";