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/offcanvas/offcanvas.js

@@ -0,0 +1,7 @@
+(function () {
+  'use strict'
+
+  document.querySelector('[data-toggle="offcanvas"]').addEventListener('click', function () {
+    document.querySelector('.offcanvas-collapse').classList.toggle('open')
+  })
+})()