docs: add shortcode for `added-in`

extract duplicate labels <small class="d-inline-flex px-2 py-1 fw-semibold text-success bg-success bg-opacity-10 rounded-2">Added in v...</small> into /site/layouts/shortcodes/:

{{- /*
  Usage: `since version`

*/ -}}

{{- $version := .Get "version" | default $.Site.version -}}

<small class="d-inline-flex px-2 py-1 fw-semibold text-success bg-success bg-opacity-10 rounded-2">Added in v{{ $version }}</small>

Originally posted by @WinterSilence in https://github.com/twbs/bootstrap/issues/35980#issuecomment-1064880886