Add page describing reference fields

**Please describe what you are requesting**
Add a page describing the function/use of `..._reference` fields

Link to this page from the schema reference fields descriptions

Start by locating the schema reference field descriptions and the existing documentation structure. Add a page explaining the function and use of `..._reference` fields, then link to it from those schema descriptions; done when readers can reach the page from each relevant field description.

Written by the indexing model from the issue text.