Initialize upgrading doc for v4.0 (#7140)

Merge pull request 7140

docs/_docs/upgrading/3-to-4.md

@@ -0,0 +1,28 @@
+---
+title: Upgrading from 3.x to 4.x
+permalink: /docs/upgrading/3-to-4/
+---
+
+Upgrading from an older version of Jekyll? A few things have changed in Jekyll 4
+that you'll want to know about.
+
+Before we dive in, you need to have at least Ruby 2.3.0 installed. Run the following
+in your terminal to check
+
+```sh
+ruby -v
+```
+
+If you're using Ruby >= 2.3.0, go ahead and fetch the latest version of Jekyll:
+
+```sh
+gem update jekyll
+```
+
+---
+
+  *Insert sections here*
+
+---
+
+*Did we miss something? Please click "Improve this page" above and add a section. Thanks!*