Document transitive closure

[peterdutey]

• [x] create a vignette/Rd explaining what therapy episodes are and the transitive closure process. Link this in the bridge_tbl functions.
• [x] explain what happens for [prescription_status] IN ('cancelled', 'draft', 'entered-in-error') (missing)
• [x] explain combinations with [prescription_status] NOT IN ('cancelled', 'draft', 'entered-in-error')
• [x] put paper 1 diagram somewhere in vignette
• [x] add a link to this documentation in roxygen chunk for .create_table_drug_prescriptions_edges(), create_therapy_episodes(), transitive_closure_control()