[Doc-SIG] Adding third-party links

[A.M. Kuchling]

A long while ago /F had a proposal for an XML format for publishing an
index to documentation so that other packages can incorporate those
links: 

	http://effbot.org/zone/idea-seealso.htm

I've dusted off some old code in sandbox/seealso and updated it, added
a hacked copy of pyspecific.py that adds a :seealsolinks:`module-name`
directive that can be put in the seealso section.  Does this seem
like an idea worth pursuing?

Potential changes: I think the node tree generated by the directive
isn't quite right; the paragraph it returns is wrapped inside another
paragraph.  Should it use Atom rather than a custom XML DTD?  And how
would it be integrated into the build process?  (I'd suggest having a
separate 'make seealso' target that fetched a list of files, parsed
them, and stored them in the build/ directory.)

--amk