Example keywords validation (#2956)

This pull request closes #2551: making sure that the keywords don't already appear in the example's title.

I also added validation that checks that they are at least two characters long, as this is enforced by the homepage when type your search.

services/date/date.service.js

@@ -46,7 +46,7 @@ module.exports = class Date extends BaseService {
         pattern: ':timestamp',
         namedParams: { timestamp: '1540814400' },
         staticPreview: this.render({ relativeDateString: '2 days ago' }),
-        keywords: ['date', 'time', 'countdown', 'countup', 'moment'],
+        keywords: ['time', 'countdown', 'countup', 'moment'],
         documentation,
       },
     ]