Create a secondary sitemap for our site search that includes all live versions

Today, the [docs sitemap](https://github.com/elastic/built-docs/blob/master/html/sitemap.xml) only includes the current version. IIRC, this was deliberate to encourage Google to weight the current version over old versions.

In our own site search, we want to be able to surface other versions, and only having current in the sitemap complicates that for the current crawler-based implementation.

If we generate a secondary sitemap that includes all of the versions we want to index for our site search, we can probably reduce some of the pain around indexing new minor releases.

This would be an interim improvement until we can replace the current crawler-based search.