Commerce docset index file

Author: AugustMiller

docs/commerce/README.md

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+# Craft Commerce
+
+This section contains documentation for each version of our first-party ecommerce plugin, [Commerce](https://craftcms.com/commerce).
+
+- [Commerce 5.x](5.x/) (Current) →
+- [Commerce 4.x](4.x/) →
+- [Commerce 3.x](3.x/) →
+- [Commerce 2.x](2.x/) →
+- [Commerce 1.x](1.x/) →