As discussed in the context of T261285, we need to provide a way of getting to the Wikibase's main page.

If we hadn't picked up this ticket an alternative could be to add a hint/documentation to https://en.wikipedia.org/w/api.php?action=query&meta=siteinfo&formatversion=2 or similar to get it from another API.

We decided to do this because the low cost of implementation seems an improvement over breaking the OpenRefine workflow/making people make additional api requests.

Key is to be named main_page.