Add a mechanism for documenting dead links

From [a conversation on Gitter](https://gitter.im/imagej/imagej.github.io?at=60b8d63814d9cd234a74ed67):

@velissarious wrote:
> Why not link to https://archive.org/web/ based on the date of the last edit and the dead link?
>
> For example this is the page with the instructions found in the dead link https://web.archive.org/web/20110123084336/http://developer.amd.com/gpu/AMDAPPSDK/downloads/Pages/default.aspx

@ctrueden wrote:
> Ah yeah, that's a good idea!
>
> What would be really slick would be something like: `{% include deadlink url='...' %}` and then it links to web.archive.org, but with a little "broken link" icon after the link, which has a tooltip explaining that this is a dead link that might have a newer live replacement, and the reader is encouraged to edit the page to improve it.

Start by locating the site's existing page templates and include mechanisms, then review how documentation links are authored. The work is done when a deadlink include can point to an archived URL, display a broken-link indicator with explanatory tooltip, and encourage readers to update the page.

Written by the indexing model from the issue text.