Remove documentation for axes_grid (matplotlib#23840)

It was removed in matplotlib#22886.

Fixes matplotlib#23560

Author: QuLogic

doc/api/index.rst

@@ -163,4 +163,3 @@ Alphabetical list of modules:
    toolkits/mplot3d.rst
    toolkits/axes_grid1.rst
    toolkits/axisartist.rst
-   toolkits/axes_grid.rst

doc/api/toolkits/axes_grid.rst

@@ -1,5 +1,7 @@
 .. module:: mpl_toolkits.axes_grid1
 
+.. redirect-from:: /api/toolkits/axes_grid
+
 ``mpl_toolkits.axes_grid1``
 ===========================
 
@@ -15,6 +17,13 @@ See :ref:`axes_grid1_users-guide-index` for a guide on the usage of axes_grid1.
    :align: center
    :scale: 50
 
+.. note::
+
+   This module contains classes and function that were formerly part of the
+   ``mpl_toolkits.axes_grid`` module that was removed in 3.6. Additional
+   classes from that older module may also be found in
+   `mpl_toolkits.axisartist`.
+
 .. currentmodule:: mpl_toolkits
 
 **The submodules of the axes_grid1 API are:**

doc/api/toolkits/axes_grid1.rst

@@ -17,6 +17,13 @@ You can find a tutorial describing usage of axisartist at the
    :align: center
    :scale: 50
 
+.. note::
+
+   This module contains classes and function that were formerly part of the
+   ``mpl_toolkits.axes_grid`` module that was removed in 3.6. Additional
+   classes from that older module may also be found in
+   `mpl_toolkits.axes_grid1`.
+
 .. currentmodule:: mpl_toolkits
 
 **The submodules of the axisartist API are:**