Add theme documentation for end-users - IDEA 66

[mattstratton]

This will be a document (stored in the root of the theme project) that contains all of the "API" documentation for the theme; what fields are available, how to use them, etc.

Effort: 6/10
Impact: 10/10

[mattstratton]

Note - I am going to create a long-lived branch for this issue, so don't kill me.

[mattstratton]

Here's the file:

https://github.com/devopsdays/devopsdays-theme/blob/documentation-updates/REFERENCE.md

The branch is documentation-updates

[bridgetkromhout]

I am going to create a long-lived branch for this issue, so don't kill me.

🤣 You know me well.