[Feature] Links relative to the repository root

[amezin]

Description

GitHub renders "absolute" links (/dir/file.md) relative to the root of the repository. This is convenient when, for example, a file is accessible from multiple locations through symlinks, with different nesting levels.

Currently such links are silently ignored.

Describe the solution you'd like

Resolve these links the same way as GitHub does. Since this behavior is specific to GitHub, it should be possible to enable/disable it with a config option.

And when resolution of absolute paths is not enabled - maybe they should be treated as errors (also a configuration option)? Otherwise, you may have potentially broken links that are silently ignored.

Describe alternatives you've considered

For now I converted all links to relative ones

[theoludwig]

Hey!
Thank you for taking the time to open this issue. @amezin

I'm not against the idea, to avoid breaking change, keep the default behavior as is, and with an option enabled (I don't know how we could call it, but I would avoid including "GitHub" in the name, as it could be used for other use cases than GitHub), we validate absolute links, where the root is where markdownlint is executed.

Feel free to open a PR, if you wish to implement this feature. 😄