Provide multiple sitemaps for specific content types

Problem/Motivation

Currently, Thunder provides one default sitemap, that contains links to pages of all content types. SEO people want to organize different page types into different sitemaps, to be able to quickly see if one specific page type has errors (according to googler search console).

While we already can create multiple sitemaps, with the simple_sitemap module, we cannot create a sitemap index, that would serve as the default sitemap for the page, which links to all the specific sitemaps.

Proposed resolution

Provide a new sitemap type for sitemap indexes.