Document the wiki_page field in the events API

What does this MR do and why?

An event whose target is a wiki page carries a wiki_page object, and this page does not mention it. API::Entities::Event exposes it under if: ->(event, _options) { event.wiki_page? }, rendered with API::Entities::WikiPageBasic, so it is format, slug, title and wiki_page_meta_id.

Nine example events are shown across the three endpoints on this page and none of them is a wiki page event, so there was no example the field could be added to without being wrong. I added a wiki page event to the "List all events" response instead, and one sentence under it saying that only wiki events carry the object.

I placed wiki_page after author in the new event, which is where the entity exposes it.

imported and imported_from were already on all nine examples, so nothing else needed changing. There are no response attribute tables on this page.

I found this while adding the same field to the Go API client, in gitlab-org/api/client-go!3050.

MR acceptance checklist

This change is documentation only. No endpoint, parameter or behaviour changes. I parsed every JSON block on the page after editing: all three still parse, the new event carries wiki_page, and the nine existing events are untouched.