Decide on the link format and add support for it

[skyne98]

1. Decide on the format for links to other units inside .md documents
   Currently the options are simplistic, but "magic" <module-name>/<unit-name>, which can become harder to parse after Allow modules to have directory structure #4, or some kind of schema based murs://<module-name>/<unit-name> or murs/<module-name>/<unit-name>.
2. Look through the markdown file during the build process and replace those links which links that lead to actual hosted files.

[skyne98]

Changed the links skyne98/murs-library@077f14e.