doc: improve installation description of siteconfig files in cookbook recipes

[xdelaruelle]

Following this mailing-list post.

Improve installation directives of the siteconfig files proposed in the cookbook recipes. etcdir may vary from one installation to another so it is wished to get clear hints on the way to find the location where to install the proposed siteconfig file.

Upcoming config sub-command may help to get this siteconfig file location.