Document results data model for modules

[anthonysena]

Related to #43, Strategus & its modules should be capable of producing documentation that fully describes the results data model. A good example of this was created by @msuchard during the SOS Challenge here: https://github.com/ohdsi-studies/FluoroquinoloneAorticAneurysm/blob/master/docs/StrategusDataModel.html.

Presuming all modules adopt the RMM package for describing & managing the results schema, perhaps this type of functionality should be supported via RMM and exposed via Strategus?

Tagging @azimov for thoughts on this.

[anthonysena]

Noting that @msuchard put together a nice document here that outlines the results data model. I'd like to make this type of artifact a part of each Strategus release to help developers working with the results data model.

[anthonysena]

Adding some notes based on the work done in #167 to document the results data model:

• Using SchemaSpy to auto-generate the results data model documentation which will reside with the Strategus documentation.
• Adopted the pkgdown workflow to automatically publish the package documentation & results data model specifications upon a release of Strategus.
• The script for updating the results data model documentation is found in extras/DocumentResultsDataModel.R.
• Table descriptions are stored in extras/rdms/table_descriptions.csv and this file is maintained manually.
• Results table relationships (i.e. foreign keys) are stored in extras/rdms/results_table_relationships.csv. This file is maintained manually. The initial version of this file was developed by extracting the SQL from the OhdsiShinyModules repo to determine the relationships between tables.