create sitemapindex.xml

[orbeckst]

Once #78 is complete, we need a top-level sitemapindex.xml file that contains links to all our sitemaps; this sitemapindex.xml would be static and we can just add it to our jekyll site.

[orbeckst]

We will have to update our search configuration https://github.com/algolia/docsearch-configs/blob/master/configs/mdanalysis.json to read

  "sitemap_urls": [
    "https://www.mdanalysis.org/sitemapindex.xml"
  ],

Notes on syntax https://docsearch.algolia.com/docs/config-file/

[orbeckst]

Will reopen until I have algolia/docsearch-configs#2091 merged.

[orbeckst]

The algolia configuration was updated.