Entity-Relationship diagram and Mdm::Module::Detail to Mdm::Module::Instance SQL translation guide

Allow creation of Entity-Relationship Diagrams (ERDs) for Mdm and Mdm::Module with rake erd:mdm (or rake erd) and rake erd:mdm:module, respectively.  Integrated png ERD of Mdm in YARD docs on the Mdm module documentation page.  Guide for translating SQL queries from Mdm::Module::Detail cache in metasploit_data_models <= 0.16.5 and Mdm::Module::Instance cache in metasploit_data_models >= 0.17.2 is available in docs/mdm_module_sql_translations.md, which is available as a static file in the generated YARD docs and linked to from the Mdm::Module docs page.