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/helpers/clearfix.md

@@ -0,0 +1,36 @@
+---
+layout: docs
+title: Clearfix
+description: Quickly and easily clear floated content within a container by adding a clearfix utility.
+group: helpers
+aliases: "/docs/5.0/helpers/"
+---
+
+Easily clear `float`s by adding `.clearfix` **to the parent element**. Can also be used as a mixin.
+
+Use in HTML:
+
+{{< highlight html >}}
+<div class="clearfix">...</div>
+{{< /highlight >}}
+
+The mixin source code:
+
+{{< scss-docs name="clearfix" file="scss/mixins/_clearfix.scss" >}}
+
+Use the mixin in SCSS:
+
+{{< highlight scss >}}
+.element {
+  @include clearfix;
+}
+{{< /highlight >}}
+
+The following example shows how the clearfix can be used. Without the clearfix the wrapping div would not span around the buttons which would cause a broken layout.
+
+{{< example >}}
+<div class="bg-info clearfix">
+  <button type="button" class="btn btn-secondary float-left">Example Button floated left</button>
+  <button type="button" class="btn btn-secondary float-right">Example Button floated right</button>
+</div>
+{{< /example >}}