DOC: Add link to previous versions documentation (#43809)

Author: DrGFreeman

doc/source/index.rst.template

@@ -12,6 +12,9 @@ pandas documentation
 
 **Download documentation**: `PDF Version <pandas.pdf>`__ | `Zipped HTML <pandas.zip>`__
 
+**Previous versions**: Documentation of previous pandas versions is available at
+`pandas.pydata.org <https://pandas.pydata.org/>`__.
+
 **Useful links**:
 `Binary Installers <https://pypi.org/project/pandas>`__ |
 `Source Repository <https://github.com/pandas-dev/pandas>`__ |