Add dark mode support

Heavily WIP still, but this begins the process of implementing dark mode for our docs and across the project itself.

- Color modes are toggled in the docs navbar with a custom toggler, which stores the select color mode in local storage.
- Color modes can also be set via data attribute thanks to `data-theme` (with light or dark options available currently).
- Docs are heavily WIP for demonstrating the dark mode.
- In order to best implement color modes, I've spiked out a number of new Sass and CSS variables (e.g., `--bs-secondary-bg` and `--bs-tertiary-bg`). In addition, I've added new global CSS variables like `--bs-border-color` and more. So, in addition to general color modes and theming support, we get greater real-time customization, too.

Todos and open questions:

- [ ] Do we refer to these as themes or color modes?
- [ ] Do we provide a color mode toggler JS plugin?
- [ ] Update all components to better utilize global CSS variables so they can be more easily themed (e.g., see `$dropdown-*` Sass variable changes in the diff).

site/assets/scss/_component-examples.scss

@@ -341,7 +341,7 @@
   position: relative;
   padding: .75rem ($bd-gutter-x * .5);
   margin-bottom: 1rem;
-  background-color: var(--bs-gray-100);
+  background-color: var(--bs-tertiary-bg);
 
   @include media-breakpoint-up(md) {
     padding: .75rem 1.25rem;
@@ -364,7 +364,7 @@
 
   pre code {
     @include font-size(inherit);
-    color: $gray-900; // Effectively the base text color
+    color: var(--bs-body-color); // Effectively the base text color
     word-wrap: normal;
   }
 }