Sanity check for internal and external links within content

[stonemaster]

Same as the compilation of the source code examples, all internal as well external links should be checked during the sanity check. That means that the external links are still reachable and not out-dated and we don't have typos in the links. A first simple check could be to extract all elements out of the final, rendered markdown and check whether we don't get 4xx or 5xx HTTP error codes for the links.