man: man pages generated via pandoc from the Markdown files
Makefile: add "nroff" target to generate nroff files

Add `config/md2nroff.pl` script to convert a libfabric Markdown man
page to an nroff man page using pandoc.  Since the Markdown man pages
are not 100%/pure Markdown (they contain some Jekyll markup for the
Github web rendering engine), we have to pre-process each Markdown
file a little and then run it through pandoc.

We also handle a few corner cases that pandoc doesn't handle well
(e.g., strip out hyperlinks from the Markdown, as pandoc doesn't know
how to render them in nroff).

Finally, md2nroff.pl is "smart" in that it will do two things:

1. Convert the @DATE@ token in the Markdown man page to the actual
   date in the generated nroff page.
1. If a Markdown page generates the same nroff output, it won't write
   out a new nroff page (i.e., it won't automatically update the date
   unless something has actually changed in the rendered nroff).