Bundle CSS with Hugo's Sass feature (#705)

* migrate bootstrap and flags to bundle

* migrated main.css

* migrate navbar.css and plyr

* migrated mulish font and darkreader

* migrated static/css/sections stylesheets

* migrated list page and sidebar styles

* migrated search.html ( nothing to add )

* migration single page styles

* remove duplicated style tags

* migrate notes.  Deleted unused css in static

* migrate 404 css to bundle

* migrate katex styles

* migrate embedpdf styles

* migrated katex css

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Co-authored-by: Aaron Qian <aaron@yeet.io>

assets/styles/override.scss

@@ -0,0 +1,15 @@
+/* override this file for custom css */
+
+/* you can import boostrap mixins */
+// @import 'bootstrap/scss/mixins';
+// Example usage
+// .some-class {
+//   @include media-breakpoint-up(sm) {
+//     // Larger than sm: 576px
+//     display: inline;
+//   }
+//   @include media-breakpoint-down(md) {
+//     // Smaller than md: 768px
+//     display: block;
+//   }
+// }