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/cover/cover.css

@@ -0,0 +1,106 @@
+/*
+ * Globals
+ */
+
+/* Links */
+a,
+a:focus,
+a:hover {
+  color: #fff;
+}
+
+/* Custom default button */
+.btn-secondary,
+.btn-secondary:hover,
+.btn-secondary:focus {
+  color: #333;
+  text-shadow: none; /* Prevent inheritance from `body` */
+  background-color: #fff;
+  border: .05rem solid #fff;
+}
+
+
+/*
+ * Base structure
+ */
+
+html,
+body {
+  height: 100%;
+  background-color: #333;
+}
+
+body {
+  display: -ms-flexbox;
+  display: flex;
+  color: #fff;
+  text-shadow: 0 .05rem .1rem rgba(0, 0, 0, .5);
+  box-shadow: inset 0 0 5rem rgba(0, 0, 0, .5);
+}
+
+.cover-container {
+  max-width: 42em;
+}
+
+
+/*
+ * Header
+ */
+.masthead {
+  margin-bottom: 2rem;
+}
+
+.masthead-brand {
+  margin-bottom: 0;
+}
+
+.nav-masthead .nav-link {
+  padding: .25rem 0;
+  font-weight: 700;
+  color: rgba(255, 255, 255, .5);
+  background-color: transparent;
+  border-bottom: .25rem solid transparent;
+}
+
+.nav-masthead .nav-link:hover,
+.nav-masthead .nav-link:focus {
+  border-bottom-color: rgba(255, 255, 255, .25);
+}
+
+.nav-masthead .nav-link + .nav-link {
+  margin-left: 1rem;
+}
+
+.nav-masthead .active {
+  color: #fff;
+  border-bottom-color: #fff;
+}
+
+@media (min-width: 48em) {
+  .masthead-brand {
+    float: left;
+  }
+  .nav-masthead {
+    float: right;
+  }
+}
+
+
+/*
+ * Cover
+ */
+.cover {
+  padding: 0 1.5rem;
+}
+.cover .btn-lg {
+  padding: .75rem 1.25rem;
+  font-weight: 700;
+}
+
+
+/*
+ * Footer
+ */
+.mastfoot {
+  color: rgba(255, 255, 255, .5);
+}

site/content/docs/4.5/examples/cover/index.html

@@ -0,0 +1,35 @@
+---
+layout: examples
+title: Cover Template
+extra_css:
+  - "cover.css"
+body_class: "text-center"
+include_js: false
+---
+
+<div class="cover-container d-flex w-100 h-100 p-3 mx-auto flex-column">
+  <header class="masthead mb-auto">
+    <div class="inner">
+      <h3 class="masthead-brand">Cover</h3>
+      <nav class="nav nav-masthead justify-content-center">
+        <a class="nav-link active" href="#">Home</a>
+        <a class="nav-link" href="#">Features</a>
+        <a class="nav-link" href="#">Contact</a>
+      </nav>
+    </div>
+  </header>
+
+  <main role="main" class="inner cover">
+    <h1 class="cover-heading">Cover your page.</h1>
+    <p class="lead">Cover is a one-page template for building simple and beautiful home pages. Download, edit the text, and add your own fullscreen background photo to make it your own.</p>
+    <p class="lead">
+      <a href="#" class="btn btn-lg btn-secondary">Learn more</a>
+    </p>
+  </main>
+
+  <footer class="mastfoot mt-auto">
+    <div class="inner">
+      <p>Cover template for <a href="https://getbootstrap.com/">Bootstrap</a>, by <a href="https://twitter.com/mdo">@mdo</a>.</p>
+    </div>
+  </footer>
+</div>