Add rivet-mkhtml task

Add a task for `rivet-mkhtml` to inspect the output of Rivet analyses.
This can alos be a useful final target, e.g. when not interested in NP correction factors, or when simply wanting to check the outputs of a rivet analysis before calculating NP correction factors.

Start by locating the existing task definitions and the task that runs Rivet analyses, then compare how external commands and final targets are represented. Add the rivet-mkhtml task so it can inspect analysis output and be used as a final target, and verify it through the project's existing task workflow.

Written by the indexing model from the issue text.