add link to former versions (#666)

Author: solegalli

docs/index.rst

@@ -267,4 +267,5 @@ Table of Contents
    contribute/index
    about/index
    whats_new/index
+   versions/index
    donate

docs/versions/index.rst

@@ -0,0 +1,12 @@
+Other versions
+==============
+
+Web-based documentation is available for versions listed below:
+
+- `Feature-engine 1.6 <https://feature-engine.trainindata.com/en/1.6.x/index.html>`_
+- `Feature-engine 1.5 <https://feature-engine.trainindata.com/en/1.5.x/index.html>`_
+- `Feature-engine 1.4 <https://feature-engine.trainindata.com/en/1.4.x/index.html>`_
+- `Feature-engine 1.3 <https://feature-engine.trainindata.com/en/1.3.x/index.html>`_
+- `Feature-engine 1.2 <https://feature-engine.trainindata.com/en/1.2.x/index.html>`_
+- `Feature-engine 1.1 <https://feature-engine.trainindata.com/en/1.1.x/index.html>`_
+- `Feature-engine 0.6 <https://feature-engine.trainindata.com/en/0.6.x_a/>`_