- Use "LinkChecker User Manual" as the source for both pages.
- .UR/.UE for external links to allow mandoc to create links in html.
- Use Linux man-pages format for cross references e.g.
.BR linkcheckerrc (5) which are replace in the html by the Makefile.
Removes the need for diff files and is a currently maintained project.
Cross references are only supported for mdoc macros but because we only
have two pages this can be achieved with sed.
A clean target is added to the Makefile to make development easier.
