DOC: Add axes_api to documentation after the refactoring

Recently, axes was converted from a module to a package.
This adds axes_api to the doctree back.
`show-inheritance` was removed because it tells the user that
`Axes` `Bases: matplotlib.axes._base._AxesBase`
which is not that relevant I think.
This is mentioned in issue matplotlib#1461

Author: Yaron de Leeuw

doc/api/axes_api.rst

@@ -6,8 +6,7 @@ axes
 :mod:`matplotlib.axes`
 ======================
 
-.. automodule:: matplotlib.axes
+.. automodule:: matplotlib.axes._axes
    :members:
    :undoc-members:
-   :show-inheritance: