build: 🔧 add explain() reference doc to website (#275)

# Description

Noticed it was missing, so I added it here. 

Needs a quick review.

## Checklist

- [X] Ran `just run-all`

Author: signekb

_quarto.yml

@@ -68,6 +68,7 @@ quartodoc:
     desc: Core functions used to check Data Packages.
     contents:
     - name: check
+    - name: explain
     - name: read_json
 
   - title: Configuration