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 #1461
DOC: changed axes_api from automodule to autoclass

with automodule we had: `matplotlib.axes._axes.Axes`
while with autoclass we have `matplotlib.axes.Axes`

However, we must remember this documents only one class, so if
classes will be added to the module, they should be added
by hand.
Merge pull request #2427 from jarondl/add-Axes-api-doc

DOC: Add axes_api to documentation after the refactoring