Bump version to 5.0.0-alpha1 (#29925)

Also add v4.5.0 in versions and keep README.md pointing to v4.5.0 so that there are no broken stuff.

site/content/docs/5.0/examples/cover/cover.css

@@ -0,0 +1,53 @@
+/*
+ * Globals
+ */
+
+
+/* Custom default button */
+.btn-secondary,
+.btn-secondary:hover,
+.btn-secondary:focus {
+  color: #333;
+  text-shadow: none; /* Prevent inheritance from `body` */
+}
+
+
+/*
+ * Base structure
+ */
+
+body {
+  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
+ */
+
+.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;
+}

site/content/docs/5.0/examples/cover/index.html

@@ -0,0 +1,34 @@
+---
+layout: examples
+title: Cover Template
+extra_css:
+  - "cover.css"
+html_class: "h-100"
+body_class: "d-flex h-100 text-center text-white bg-dark"
+include_js: false
+---
+
+<div class="cover-container d-flex w-100 h-100 p-3 mx-auto flex-column">
+  <header class="mb-auto">
+    <div>
+      <h3 class="float-md-left mb-0">Cover</h3>
+      <nav class="nav nav-masthead justify-content-center float-md-right">
+        <a class="nav-link active" aria-current="page" href="#">Home</a>
+        <a class="nav-link" href="#">Features</a>
+        <a class="nav-link" href="#">Contact</a>
+      </nav>
+    </div>
+  </header>
+
+  <main class="px-3">
+    <h1>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 font-weight-bold border-white bg-white">Learn more</a>
+    </p>
+  </main>
+
+  <footer class="mt-auto text-white-50">
+    <p>Cover template for <a href="https://getbootstrap.com/" class="text-white">Bootstrap</a>, by <a href="https://twitter.com/mdo" class="text-white">@mdo</a>.</p>
+  </footer>
+</div>