Update new docs version picker to work on home and examples pages (#36245)

* Update new docs version picker to work on home and examples pages

* Add addd key to opacity page

* Add more added frontmatter

* Update picker to fix migration guide links, but still work for docs and examples

site/content/docs/5.1/helpers/stacks.md

@@ -4,6 +4,7 @@ title: Stacks
 description: Shorthand helpers that build on top of our flexbox utilities to make component layout faster and easier than ever.
 group: helpers
 toc: true
+added: "5.1"
 ---
 
 Stacks offer a shortcut for applying a number of flexbox properties to quickly and easily create layouts in Bootstrap. All credit for the concept and implementation goes to the open source [Pylon project](https://almonk.github.io/pylon/).

site/content/docs/5.1/helpers/vertical-rule.md

@@ -4,6 +4,7 @@ title: Vertical rule
 description: Use the custom vertical rule helper to create vertical dividers like the `<hr>` element.
 group: helpers
 toc: true
+added: "5.1"
 ---
 
 ## How it works

site/content/docs/5.1/utilities/opacity.md

@@ -3,6 +3,7 @@ layout: docs
 title: Opacity
 description: Control the opacity of elements.
 group: utilities
+added: "5.1"
 ---
 
 The `opacity` property sets the opacity level for an element. The opacity level describes the transparency level, where `1` is not transparent at all, `.5` is 50% visible, and `0` is completely transparent.