Don't install empty man page directories

The CMake logic for installing man-page link files would create a
directory for each section that might possibly contain links.  Now it
only creates directories for the sections that actually contain links.
This also fixes a bug when using CMake 2.6, where the docs/links
directory needed to be present, even if your project has no links
whatsoever.  It's now fine for the directory to be missing.
Explicit header and footer in man pages

You now need to explicitly provide the text that goes in the header and
footer of each man page.  We were relying on a pandoc default before,
but that default has changed recently, so now we're requiring you to
provide specific text.
Remove old Sphinx documentation template

To complete the move to man page documentation.
Merge branch 'feature/docs' into c-template

* feature/docs:
  Remove old Sphinx documentation template
  Explicit header and footer in man pages
  Don't install empty man page directories