fix: update href formatter to handle mdx correctly

[bdatdo0601]

Greetings,

The link transformer have a bug where foobar.mdx href will be foobarx which cause an errors on routing. I updated the regex and tested locally with md and mdx. Let me know if there is a preferred logic that you would like to implement instead

Thank you for maintaining this :)

[phartenfeller]

Thanks a lot for the fix!