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/components/close-button.md

@@ -0,0 +1,18 @@
+---
+layout: docs
+title: Close button
+description: A generic close button for dismissing content like modals and alerts.
+group: components
+---
+
+**Be sure to include text for screen readers**, as we've done with `aria-label`. Disabled close buttons have `pointer-events: none` applied to, preventing hover and active states from triggering.
+
+{{< example >}}
+<button type="button" class="close" aria-label="Close">
+  <span aria-hidden="true">&times;</span>
+</button>
+
+<button type="button" class="close" disabled aria-label="Close">
+  <span aria-hidden="true">&times;</span>
+</button>
+{{< /example >}}