Add project trends graph to documentation

[Letme]

Draw project trends graph (which you can see in Coverity interface) is a line graph, which displays number of defects (for example new, outstanding, resolved, fixed, triaged and/or issue density) on the time scale for duration of the project.

It requires creation of new sphinx directive, where we again specify the :col: (columns to be drawn as lines on the chart) and optional :starttime: and :endtime: which define a time scale (default is first() and last()). It will also require to dig deeper into the Coverity interface to extract this data out.

Requested by @dryodon