Add supporting themes required for Lotusdocs

themes/twbs/bootstrap/scss/mixins/_banner.scss

@@ -0,0 +1,7 @@
+@mixin bsBanner($file) {
+  /*!
+   * Bootstrap #{$file} v5.3.8 (https://getbootstrap.com/)
+   * Copyright 2011-2025 The Bootstrap Authors
+   * Licensed under MIT (https://github.com/twbs/bootstrap/blob/main/LICENSE)
+   */
+}