Relative links are broken in markdown

[mattpolzin]

When generating markdown, especially for use in GitHub, I like to rely on relative links as a robust way to link between documentation files living at file paths consistent relative to each other.

When I run the current GitHub Action, the default baseURL is used ("/"). This results in broken links between documentation files within GitHub or the GitHub Wiki.

I think this is partially solved by exposing the --base-url argument via the GitHub action (should this also default to "./" to coincide with the default format of .commonmark?).

That is only half the solution, though, because setting the baseURL to "./" results in Links prefixed with "/./". The solution to the second part of this appears to be straight forward enough, but time, experience, and feedback from maintainers will prove or disprove that!

[mattpolzin]

I've got this WIP. Will PR soon.