doc: Add changing RTD version to checklist (pytest-dev#9606)

I still think it makes sense to always serve docs from the release branch (rather than the latest tagged release), but that means we need to update it on new releases.

RELEASING.rst

@@ -160,6 +160,8 @@ Both automatic and manual processes described above follow the same steps from t
        git tag MAJOR.{MINOR+1}.0.dev0
        git push upstream MAJOR.{MINOR+1}.0.dev0
 
+#. For major and minor releases, change the default version in the `Read the Docs Settings <https://readthedocs.org/dashboard/pytest/advanced/>`_ to the new branch.
+
 #. Send an email announcement with the contents from::
 
      doc/en/announce/release-<VERSION>.rst