[Rivet] Rivet Doxygen now based on release tag

[Andy Buckley]

Since Frank raised the issue, I changed the Rivet Doxygen build to be
based on the 2.1.1 release tag and have also updated the stored
code/2.1.1 version to correctly point at that tag.

When a new release is made someone now needs to cd into the
~/rivet/codedocs/code directory and call "hg update rivet-2.X.Y" before
running the Doxygen building scripts, copying to the versioned dirs,
etc.  I've updated the release instructions accordingly:
https://rivet.hepforge.org/trac/wiki/ReleaseProcedure

Andy

-- 
Dr Andy Buckley, Royal Society University Research Fellow
Particle Physics Expt Group, University of Glasgow / PH Dept, CERN