Switch to Hugo

This commit includes all the needed workarounds and most changes from the main branch for everything to work, like:

* removing empty lines in raw HTML that break output
* read .browserslistrc, CSS variables from disk instead of duplicating it
* using Hugo mounts
* using Hugo for the docs CSS/JS
* move ToC Sass code to a separate file while adapting it for Hugo

Thus, this patch makes our npm scripts faster since lint runs on one step and there's no separate docs assets processing.

site/content/docs/4.5/examples/carousel/carousel.css

@@ -0,0 +1,90 @@
+/* GLOBAL STYLES
+-------------------------------------------------- */
+/* Padding below the footer and lighter body text */
+
+body {
+  padding-top: 3rem;
+  padding-bottom: 3rem;
+  color: #5a5a5a;
+}
+
+
+/* CUSTOMIZE THE CAROUSEL
+-------------------------------------------------- */
+
+/* Carousel base class */
+.carousel {
+  margin-bottom: 4rem;
+}
+/* Since positioning the image, we need to help out the caption */
+.carousel-caption {
+  bottom: 3rem;
+  z-index: 10;
+}
+
+/* Declare heights because of positioning of img element */
+.carousel-item {
+  height: 32rem;
+}
+.carousel-item > img {
+  position: absolute;
+  top: 0;
+  left: 0;
+  min-width: 100%;
+  height: 32rem;
+}
+
+
+/* MARKETING CONTENT
+-------------------------------------------------- */
+
+/* Center align the text within the three columns below the carousel */
+.marketing .col-lg-4 {
+  margin-bottom: 1.5rem;
+  text-align: center;
+}
+.marketing h2 {
+  font-weight: 400;
+}
+.marketing .col-lg-4 p {
+  margin-right: .75rem;
+  margin-left: .75rem;
+}
+
+
+/* Featurettes
+------------------------- */
+
+.featurette-divider {
+  margin: 5rem 0; /* Space out the Bootstrap <hr> more */
+}
+
+/* Thin out the marketing headings */
+.featurette-heading {
+  font-weight: 300;
+  line-height: 1;
+  letter-spacing: -.05rem;
+}
+
+
+/* RESPONSIVE CSS
+-------------------------------------------------- */
+
+@media (min-width: 40em) {
+  /* Bump up size of carousel content */
+  .carousel-caption p {
+    margin-bottom: 1.25rem;
+    font-size: 1.25rem;
+    line-height: 1.4;
+  }
+
+  .featurette-heading {
+    font-size: 50px;
+  }
+}
+
+@media (min-width: 62em) {
+  .featurette-heading {
+    margin-top: 7rem;
+  }
+}