Make cddlib support SAGE_SPKG_INSTALL_DOCS

[kcrisman]

#13026 recently updated and cleaned up cddlib. There is actually significant documentation currently deleted. The ps and dvi files do take up a lot of room, but if we just included the TeX sources it would be negligible in size.

This ticket is for including those few docs of cddlib and allowing them to be built and installed via the SAGE_SPKG_INSTALL_DOCS environment variable. Or whatever other solution appears if that doesn't work out so well. See #10823 for background, and #11197 for the current status of this variable.

CC: @jasongrout

Component: packages: standard

Keywords: sd40.5

Issue created by migration from https://trac.sagemath.org/ticket/13056

[kcrisman]

Changed keywords from none to sd40.5