Improve Google Search

[Paradyx]

The website is missing a sitemap at https://rpg-librarium.de/storage/public/sitemap-index.xml

[sudo42]

There already is a generated sitemap at https://rpg-librarium.de/sitemap.xml.
I think, we should instead fix some minor issues with the page structure (eg. home.md) and create a robots.txt instead.

[Paradyx]

the https://rpg-librarium.de/storage/public/sitemap-index.xml is apparently crawled automatically by google. Although the other sitemap was added manually, further research is required.