Importers: Consider moving support doc link from section header area

The "Find out what we currently support" link under Tools -> Import (simple sites only) contains a lot of information, but not a concise list of supported importers. The importers page itself acts as a better guide to what imports we support.

At the least, this link should move to the bottom of the page rather than taking up a prominent spot at the top. It should also open in a new tab or window, and we should considering adding tracking information so we know how often it gets clicked.

Start at Tools → Import (simple sites only) and inspect the “Find out what we currently support” link and the importer page it opens. Move the link to the bottom, make it open in a new tab or window, and determine whether click tracking is in scope; done means the placement and link behavior match the issue’s requirements.

Written by the indexing model from the issue text.