docs: allow writing man pages in markdown

docs: allow writing man pages in markdown

Some of the docs/misc documents are written in markdown language.
As an effort to cleanup man pages these documents will be converted into
man pages. To avoid some more conversion, add rules to the docs/Makefile
to generate man pages out of markdown files as well as pod ones.

However, pandoc doesn't know how to convert man pages links. Thus the
man links in markdown pages won't work.

Signed-off-by: Cédric Bosdonnat <cbosdonnat@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>