Add new docs Versions page.

site/_data/docs-versions.yml

@@ -0,0 +1,41 @@
+- group: v1.x
+  baseurl: https://getbootstrap.com
+  description: Every minor and patch release from v1 is listed below.
+  versions:
+    - v: 1.0.0
+    - v: 1.1.0
+    - v: 1.1.1
+    - v: 1.2.0
+    - v: 1.3.0
+    - v: 1.4.0
+
+- group: v2.x
+  baseurl: https://getbootstrap.com
+  description: Every minor and patch release from v2 is listed below.
+  versions:
+    - v: 2.0.0
+    - v: 2.0.1
+    - v: 2.0.2
+    - v: 2.0.3
+    - v: 2.0.4
+    - v: 2.1.0
+    - v: 2.1.1
+    - v: 2.2.0
+    - v: 2.2.1
+    - v: 2.2.2
+    - v: 2.3.0
+    - v: 2.3.1
+    - v: 2.3.2
+
+- group: v3.x
+  baseurl: https://getbootstrap.com/docs
+  description: Single docs site for our previous major release. Last update was v3.3.7.
+  versions:
+    - v: 3.3
+
+- group: v4.x
+  baseurl: https://getbootstrap.com/docs
+  description: Current major release with two minor releases. Last update was v4.1.3.
+  versions:
+    - v: 4.0
+    - v: 4.1