Enable intersphinx links

[keewis]

At the moment, the documentation of cftime provides a intersphinx file (https://unidata.github.io/cftime/objects.inv), but it does not contain entries for cftime.datetime and other attributes of the module:

$ python -m sphinx.ext.intersphinx https://unidata.github.io/cftime/objects.inv
std:doc
	api                                      API                                     : api.html
	index                                    cftime                                  : index.html
	installing                               Installation                            : installing.html
std:label
	genindex                                 Index                                   : genindex.html
	modindex                                 Module Index                            : py-modindex.html
	search                                   Search Page                             : search.html

This causes broken links and sphinx warnings in projects that try to link to it (e.g. xarray).

The easiest way I know to fix this would be to remove :noindex: from docs/api.rst.

[jswhit]

Pull request would be very much appreciated