DOC: add new page 'how to read these doc' in get started section

This page explains the code block are run in notebook

closes #39130

Author: rhuille

doc/source/getting_started/how_to_read_these_docs.rst

@@ -0,0 +1,41 @@
+.. _overview:
+
+{{ header }}
+
+**********************
+How to read these docs
+**********************
+
+In these tutorials you will see input code inside code blocks such as:
+
+::
+
+    import pandas as pd
+    pd.DataFrame({'A': [1, 2, 3]})
+
+
+or:
+
+.. ipython:: python
+
+    import pandas as pd
+    pd.DataFrame({'A': [1, 2, 3]})
+
+
+
+The first block is a standard python input, while in the second the `In [1]:` indicates the input is inside a `notebook <https://jupyter.org>`__. It does not make any difference **except the last line will be printed**.
+
+
+For example:
+
+.. ipython:: python
+
+    a = 1
+    a
+
+is equivalent to:
+
+::
+
+    a = 1
+    print(a)

doc/source/getting_started/index.rst

@@ -682,6 +682,7 @@ material is enlisted in the community contributed :ref:`communitytutorials`.
 
     install
     overview
+    how_to_read_these_docs
     intro_tutorials/index
     comparison/index
     tutorials