Improve documentation around using multiple sitemaps

The docs changes from the other issue.

I fixed the documentation issue of the nomenclature sometimes being 'sitemaps' and sometimes 'variants' and explained using the sitemap index some more. Sorry for not using your MR - I find the structure a bit hard to follow.

Thanks for committing some improvements to the docs.

I ended up turning the steps from my MR into a blog post: https://www.damienmckenna.com/2024/03/grouping-simple-sitemap-by-content...

If you'd like it as rewritten as a HOWTO docs page with screenshots just let me know, I'm happy to put it together.

Automatically closed - issue fixed for 2 weeks with no activity.