add config file for tmpfiles.d
    
    This config allows you to easily set up old files pruning in all
    directories that Taskotron currently uses. I assume it will be mainly
    helpful for developers. Includes documentation update and some
    formatting fixes.
    
    This also slightly changes mockchain location in Makefile to make sure
    we use something specific to Taskotron and not generic across different
    applications/use cases.
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D298