Support for stubs of deleted pages

[cl8n]

in cases where an article is completely removed with no redirect, I think it would be good to replace the article's content with something like "This article has been deleted because [...]", and link away to other relevant places if appropriate. these "deleted" articles should be special in not appearing in search results, the sitemap, etc. -- the only purpose would be to avoid breaking longstanding links elsewhere

low priority ofc but this should be simple to implement web-side.

see #12142 and the removal of .db articles for examples