Write a specification for the encoding scheme

[johngrimes]

It would be good if we could have a page in our documentation devoted to a full description of how the Pathling encoding scheme works, and what the expectations are for data that claims compatibility to this format.

This would also help us to contribute to the emerging "Columnar Schema Guidance" section of the SQL on FHIR specification: https://build.fhir.org/ig/FHIR/sql-on-fhir-v2/columnar_schema_guidance.html