config: Generate a sitemap and highlight it in robots.txt

[jwflory]

This is a Search Engine Optimization (S.E.O.) tweak. A sitemap is a file for search engines that creates a digital map of all the content published on a website. Using a plugin, Jekyll will create one for fossrit.github.io and put it at the root of the domain. Using a robots.txt is a more explicit way we tell other search engines that a sitemap is present and to go and use it for indexing.

[jwflory]

@whenbellstoll Thank ya sir! Merging. :clapper: