Add last_edit to document json

Having the last edit date of a document helps us to find old/outdated pages. The document json gets called from [KumaScript macros](https://developer.mozilla.org/en-US/docs/Template:NeedsUpdateInSection) and the "modified" field only tells us when the page was last rebuilt (which was last year at least). This PR now adds the real last edit date from a human who created the last revision to the document.

I guess adding a new field will need a rebuild of all the documents. Curious what @lmorchard thinks.