Add minimal set of links to the ROOT page

[katilp]

The CODP pages (e.g. docker guide) will link to this page for more information and further links.

Some cleaning of this page would be required. For the moment, a link to the root tutorial page can be added https://cms-opendata-workshop.github.io/workshop2021-lesson-preexercise-cpp-and-root/ although it is missing the simplest use case, i.e. opening an existing root file and exploring the contents with TBrowser.

For the record, the new CODP getting started with 2015 data page (cernopendata/opendata.cern.ch#3165) is doing this, i.e. it will show how to open a file produced by POET and how to plot pt distribution of electrons: http://opendata.cern.ch/docs/cms-getting-started-2015

[katilp]

We need to think about how to best convey the information about the practicalities in the ODG, e.g. on this page how to use the root container. The reader of this page would be happy to know that a root container exists but we only talk about it in the workshop tutorial https://cms-opendata-workshop.github.io/workshop2022-lesson-docker/03-docker-for-cms-opendata/index.html

The ODG mentions only the CMSSW containers in https://cms-opendata-guide.web.cern.ch/tools/docker/