EventPage: Add documentation link in event page

[rebeccamichelson]

Since Hackpad is closing down, let's change the "hackpad link" to "documentation" in the Events pages. Example: https://bostoncivic.media/events/civic-media-impact-and-assessmentlesspgreaterapril-2016lesspgreater.

[jpailthorpe]

With the updated design, we did not include entries to these documentation links. That said, you can easily add links to these documents via the long description markdown field under the event. If you bold and hyperlink it, it will look like this:

you can of course title the name of the link anything you want and link to any url (here I am linking to the hackpad backup google doc in the drive).