Enable doctesting for all narrative documentations

[bsipocz]

Some modules still don't have doctest enabled on their narrative docs. Quick grep for doctest-skip-all brings up this list (some may be irrelevant for doctesting, I'll remove those from the list later).

• docs/api.rst
• docs/astrometry_net/astrometry_net.rst
• docs/besancon/besancon.rst
• docs/cosmosim/cosmosim.rst
• docs/gaia/gaia.rst
• docs/image_cutouts/first/first.rst
• docs/image_cutouts/image_cutouts.rst
• docs/ipac/irsa/ibe/ibe.rst
• docs/mocserver/mocserver.rst
• docs/query.rst
• docs/simbad/simbad.rst
• docs/testing.rst
• docs/ukidss/ukidss.rst
• docs/utils.rst
• docs/utils/tap.rst
• docs/vamdc/vamdc.rst
• docs/vizier/vizier.rst
• docs/vo_conesearch/client.rst
• docs/vo_conesearch/validator.rst
• docs/vo_conesearch/vo_conesearch.rst
• docs/vsa/vsa.rst
• docs/wfau/wfau.rst