CMS: create a new 2015 getting started page

[katilp]

Create a new docs/cms-getting-started-2015 page as in http://opendata.cern.ch/docs/cms-getting-started-2011

To be changed, compared to the 2011 one:

• change order container/VM
• CMSSW_5_3_32 -> CMSSW_7_6_7
• root input file
• change Option A/B to POET, ideally, it should link to the CMS open data guide

[katilp]

Leave out the plain root step, at this point, it brings no value.

Point to (or preferably include in the CMS open data guide), the MiniAOD docs from https://twiki.cern.ch/twiki/bin/view/CMSPublic/WorkBookMiniAOD2015 and go directly with POET 2015 MINIAOD branch.