Accept date in badges. Fix errors in build.

Our guidelines suggest adding a date when using the badge markdoc
component, but until now the component's schema didn't have a defintion
for the date parameter, so it was reporting errors.

The date doesn't affect how the badge is displayed on the site, but
we can use it to see when a 'New in' badge is old enough to no longer be
relevant (typically >2yrs) so we can remove it.

@theme/markdoc/schema.ts

@@ -122,6 +122,10 @@ export const badge: Schema & { tagName: string } = {
     href: {
       type: 'String',
       required: false
+    },
+    date: { // Not displayed, but useful for knowing how old an 'updated' badge is
+      type: 'String',
+      required: false
     }
   },
   render: 'Badge'