:handshake: Update Wagtail links (#237)

Updates the domain name, and splits out our website, dev docs, and user docs.

README.md

@@ -487,9 +487,11 @@ _Wagtail, the powerful CMS for modern websites._
 
 ### Wagtail Resources
 <!--lint disable double-link-->
-- [Official Documentation](https://wagtail.io/)
+- [Official website](https://wagtail.org/)
+- [Developer documentation](https://docs.wagtail.org/en/stable/)
+- [User documentation](https://guide.wagtail.org/en-latest/)
 - [Wagtail Source Code](https://github.com/wagtail/wagtail/)
 - [awesome-wagtail](https://github.com/springload/awesome-wagtail)
-- [This week in Wagtail](https://wagtail.io/this-week-in-wagtail/) - A (most) weekly email with updates from the Wagtail core team.
+- [This week in Wagtail](https://wagtail.org/this-week-in-wagtail/) - A (most) weekly email with updates from the Wagtail core team.
 - [Wagtail Space](https://www.wagtail.space/) - Wagtail CMS events around the world.
 <!--lint enable double-link-->