Default error output does not include link to documentation

[khiga8]

The default error message for markdown-cli2 does not include description, aka the optional URL that we pass into the URL to provide consumers with more context about the rule.

It looks like as though we'd need projects to configure a separate formatter like formatter-pretty if we want the error message to link to the description URL. I don't like that we need to install a separate dependency, but this is the more idiomatic route.

Another option is to just include the URL as part of the error message so the formatter doesn't need to be configured but this is less idiomatic.

[khiga8]

On second though, I think we'll want to go the first route of recommending the configuration. This route will ensure the doc URL is provided for the native markdownlint URLs too.