Revert "Update extending.md"
Revert "Update extending.md"

This reverts commit 64b0d4cf068a9d84f8bf74275ffd3f3c84e83ab2.

netlify/netlify-cms,netlify/netlify-cms,Aloomaio/netlify-cms,netlify/netlify-cms,Aloomaio/netlify-cms,netlify/netlify-cms