Documentation lacks top-level list of executable scripts

The docs do not currently provide an overview of all the executable scripts. 
The fits-based scripts ``fitsheader``, ``fitscheck`` and ``fitsdiff`` are covered in  [docs/io/fits/usage/scripts.rst](../tree/master/docs/io/fits/usage/scripts.rst), but there is no mention anywhere of the scripts outside the fits sub-package (``samp_hub``, ``volint`` and ``wcslint``).

The docs should contain a top-level list of all scripts found under [astropy/scripts](../tree/master/scripts) with links to more-specific descriptions in each of the sub-package docs.
This gives a warning in the sphinx docs build and makes travis-ci fail:
https://travis-ci.org/astropy/astropy/jobs/27643378#L3011

In Astropy you have to use double-backticks for things that you'd like to mark up and that don't resolve as Sphinx references.