Add terminology mapping page, information on source specs for logical models, and misc. terminology clean-up

[masnick]

This PR:

1. Adds a new Terminology Mapping page and corresponding menu item under "Data Models and Exchange". This page includes the strategy used for the terminology mapping, as well as a list of the specs used in the mappings.
2. Adds a note to the top of the credential logical models on the source specifications, linking back to the relevant section in (1).
3. Includes miscellaneous terminology-related cleanup, including resolving a number if QA issues.

/cc @nkarip @litlfred