DOC: remove obsolete oldnumeric/numarray sections from reference guide.

Also fix a duplicate entry for numpy.testing due to incorrect
header underlining.
DOC: fix examples in f2py documentation.  They didn't appear at all in html.

Also fix example in testing that Sphinx saw as section header.
DOC: add NEPs overview to documentation.
DOC: fix reST formatting of npy-format NEP.
DOC: clean up docs in userguide for interfacing to compiled code a bit.

Add a link to f2py docs, which was missing.

[ci skip]
Merge pull request #4632 from rgommers/refguide-update

Doc update: NEPs, f2py examples and refguide section fix.