Indicating when extensions exist

[timgdavies]

In order to better implement the extensions mechanism of OCDS, it would be useful to surface information on extensions that exist, both in a specific page of the documentation, and potentially at contextually relevant parts of the Reference page.

For example, if you are looking at the documents block and a documents extension existed, the interface should be able to tell you this.

(Adding this issue now to log this for when we next work on standard collaborator)