Add support for mailto links #65
Description
Currently, mailto links get treated as local links which are then errored out, for example here.
error:
Documentation/user-guides/getting-started.md:4,7 (2 occurrences): link tectonic-alpha-feedback@coreos.com, normalized to: /Users/saswatamukherjee/web/prometheus-operator/Documentation/user-guides/tectonic-alpha-feedback@coreos.com: file not found