build: Rename markdown MPI manpages

A number of the markdown-ized man pages in the MPI man3 section
did not follow the expected naming convention, so rename them
to have the suffix .3.md instead of just .md.  The old names
resulted in two problems: first, they were never instaleld and
second, make maintainer-clean would delete them.  Renaming
the files to expected format fixes both issues.

Signed-off-by: Brian Barrett <bbarrett@amazon.com>

Author: bwbarrett