DOC: Add 2.6.0 to documentation archive

Author: snowman2

docs/past_versions.rst

@@ -1,6 +1,7 @@
 Documentation Archive
 =====================
 
+- `2.6.0 <https://pyproj4.github.io/pyproj/v2.6.0rel/>`_
 - `2.5.0 <https://pyproj4.github.io/pyproj/v2.5.0rel/>`_
 - `2.4.2 <https://pyproj4.github.io/pyproj/v2.4.2rel/>`_
 - `2.4.1 <https://pyproj4.github.io/pyproj/v2.4.1rel/>`_