Generate a geard.8 manpage for Debian packaging

I have created a Debian package for gear and the Debian policy requires
daemon to ship with a manpage.

This patch:
* extracts out geard program output out of index.rst to a new geard.rst
  including it at the original place for convenience.
* configure sphinx to generate a geard.8 manpage from geard.rst
* geard.rst remains unlinked in the HTML documentation since it is
  included in index.rst.

Minor issue:
* I haven't found how to add cross references between the two manpages
  without interfering with the HTML documentation output.

Change-Id: I2576cfa4acb0b91d789dc1ce42299ae91c6f9422
Merge "Generate a geard.8 manpage for Debian packaging"