Add README / ModuleOverview to JetMonitoring package
This MR is a first attempt at adding a README to the various packages/modules used with the (offline) JetMonitoring framework in Run3. It is one form of documentation that is requested in ATR-21328, the remaining parts will be continued/completed in the OfflineJetMonitoring twiki.
(also, a small comment typo fix in JetMonitoringConfig.py)