Consolidate current CDL specification(s) into NUG v2.0

[ethanrd]

CDL is described in three locations: the current NUG v1.1; the man page for the ngen3 utility; and the man page for the ngen utility. The ngen man page is, I believe, the most up to date. However, there are a few things in the current NUG CDL page that don't, and I believe should, appear in the ngen man page.

My plan is to use the text from the ngen man page as the basis for the NUG v2.0 CDL page and then go through the other two versions and make sure there isn't anything missing.

@DennisHeimbigner - Does that sound OK? I will also create a branch in the netCDF-C repo to remove that text in the two man pages and insert links to the NUG v2.0 pages that we can merge in when NUG 2.0 is good to go.