Document standard transaction/span types

Let's move [this public document](https://docs.google.com/spreadsheets/d/1SmWeX5AeqUcayrArUauS_CxGgsjwRgMYH4ZY8yQsMhQ/edit#gid=644582948) to the documentation to improve discovery, introduce versioning, and improve version control.

I propose adding these to the data model documentation but open to suggestions.