Sitemaps: Add XML sitemaps functionality to WordPress.
While web crawlers are able to discover pages from links within the site and from other sites, XML sitemaps supplement this approach by allowing crawlers to quickly and comprehensively identify all URLs included in the sitemap and learn other signals about those URLs using the associated metadata.
See https://make.wordpress.org/core/2020/06/10/merge-announcement-extensible-core-sitemaps/ for more details.
This feature exposes the sitemap index via /wp-sitemap.xml and exposes a variety of new filters and hooks for developers to modify the behavior. Users can disable sitemaps completely by turning off search engine visibility in WordPress admin.
This change also introduces a new esc_xml() function to escape strings for output in XML, as well as XML support to wp_kses_normalize_entities().
Props Adrian McShane, afragen, adamsilverstein, casiepa, flixos90, garrett-eclipse, joemcgill, kburgoine, kraftbj, milana_cap, pacifika, pbiron, pfefferle, Ruxandra Gradina, swissspidy, szepeviktor, tangrufus, tweetythierry.
Fixes #50117.
See #3670. See #19998.