Sanity Functions examples, and fix for broken 'create document' button

We've added a number of new /examples to demonstrate the use of Sanity Functions across various use-cases, such as building a Slack notifier to flag when new content is published, or adjusting the tone of documents to be consistent. Take a look at the available functions on GitHub, or check them out in the exchange.

• Fixes an issue where 'Create new document' button would wrongly appear in lists where no templates were available to create documents from.