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/floating-labels/floating-labels.css

@@ -0,0 +1,130 @@
+html,
+body {
+  height: 100%;
+}
+
+body {
+  display: -ms-flexbox;
+  display: flex;
+  -ms-flex-align: center;
+  align-items: center;
+  padding-top: 40px;
+  padding-bottom: 40px;
+  background-color: #f5f5f5;
+}
+
+.form-signin {
+  width: 100%;
+  max-width: 420px;
+  padding: 15px;
+  margin: auto;
+}
+
+.form-label-group {
+  position: relative;
+  margin-bottom: 1rem;
+}
+
+.form-label-group input,
+.form-label-group label {
+  height: 3.125rem;
+  padding: .75rem;
+}
+
+.form-label-group label {
+  position: absolute;
+  top: 0;
+  left: 0;
+  display: block;
+  width: 100%;
+  margin-bottom: 0; /* Override default `<label>` margin */
+  line-height: 1.5;
+  color: #495057;
+  pointer-events: none;
+  cursor: text; /* Match the input under the label */
+  border: 1px solid transparent;
+  border-radius: .25rem;
+  transition: all .1s ease-in-out;
+}
+
+.form-label-group input::-webkit-input-placeholder {
+  color: transparent;
+}
+
+.form-label-group input::-moz-placeholder {
+  color: transparent;
+}
+
+.form-label-group input:-ms-input-placeholder {
+  color: transparent;
+}
+
+.form-label-group input::-ms-input-placeholder {
+  color: transparent;
+}
+
+.form-label-group input::placeholder {
+  color: transparent;
+}
+
+.form-label-group input:not(:-moz-placeholder-shown) {
+  padding-top: 1.25rem;
+  padding-bottom: .25rem;
+}
+
+.form-label-group input:not(:-ms-input-placeholder) {
+  padding-top: 1.25rem;
+  padding-bottom: .25rem;
+}
+
+.form-label-group input:not(:placeholder-shown) {
+  padding-top: 1.25rem;
+  padding-bottom: .25rem;
+}
+
+.form-label-group input:not(:-moz-placeholder-shown) ~ label {
+  padding-top: .25rem;
+  padding-bottom: .25rem;
+  font-size: 12px;
+  color: #777;
+}
+
+.form-label-group input:not(:-ms-input-placeholder) ~ label {
+  padding-top: .25rem;
+  padding-bottom: .25rem;
+  font-size: 12px;
+  color: #777;
+}
+
+.form-label-group input:not(:placeholder-shown) ~ label {
+  padding-top: .25rem;
+  padding-bottom: .25rem;
+  font-size: 12px;
+  color: #777;
+}
+
+.form-label-group input:-webkit-autofill ~ label {
+  padding-top: .25rem;
+  padding-bottom: .25rem;
+  font-size: 12px;
+  color: #777;
+}
+
+/* Fallback for Edge
+-------------------------------------------------- */
+@supports (-ms-ime-align: auto) {
+  .form-label-group {
+    display: -ms-flexbox;
+    display: flex;
+    -ms-flex-direction: column-reverse;
+    flex-direction: column-reverse;
+  }
+
+  .form-label-group label {
+    position: static;
+  }
+
+  .form-label-group input::-ms-input-placeholder {
+    color: #777;
+  }
+}

site/content/docs/4.5/examples/floating-labels/index.html

@@ -0,0 +1,33 @@
+---
+layout: examples
+title: Floating labels example
+extra_css:
+  - "floating-labels.css"
+include_js: false
+---
+
+<form class="form-signin">
+  <div class="text-center mb-4">
+    <img class="mb-4" src="/docs/{{< param docs_version >}}/assets/brand/bootstrap-solid.svg" alt="" width="72" height="72">
+    <h1 class="h3 mb-3 font-weight-normal">Floating labels</h1>
+    <p>Build form controls with floating labels via the <code>:placeholder-shown</code> pseudo-element. <a href="https://caniuse.com/css-placeholder-shown">Works in latest Chrome, Safari, Firefox, and IE 10/11 (prefixed).</a></p>
+  </div>
+
+  <div class="form-label-group">
+    <input type="email" id="inputEmail" class="form-control" placeholder="Email address" required autofocus>
+    <label for="inputEmail">Email address</label>
+  </div>
+
+  <div class="form-label-group">
+    <input type="password" id="inputPassword" class="form-control" placeholder="Password" required>
+    <label for="inputPassword">Password</label>
+  </div>
+
+  <div class="checkbox mb-3">
+    <label>
+      <input type="checkbox" value="remember-me"> Remember me
+    </label>
+  </div>
+  <button class="btn btn-lg btn-primary btn-block" type="submit">Sign in</button>
+  <p class="mt-5 mb-3 text-muted text-center">&copy; 2017-{{< year >}}</p>
+</form>