Include opted-in abstract articles in the wiki's standard sitemap generation.

WMF wikis emit sitemaps via the generateSitemap.php maintenance script (or equivalent scheduled job), and that generation reads from the page table to enumerate the wiki's content. Opted-in abstract articles do not have rows in the page table by default — they are virtual pages rendered via the ShowMissingArticle hook — which means they will be absent from the generated sitemap unless sitemap generation is taught about them. The expected shape is a hook (or a direct patch to the sitemap generation path, if a hook does not already exist) that appends the opted-in entries to the sitemap after the page-table enumeration, using the same URL format and change-frequency/priority conventions as normal mainspace articles. If no suitable extension point exists in the sitemap generation code, surfacing a new one is the work covered by this sub-bullet; if it does, the work is just wiring.