Warn when linking to a .md file

[raboof]

'normal' links take a .html parameter ([foo](foo.html)), while @ref links and ToC entries expect [foo](foo.md).

This can be easy to mix up: @refs and ToC entries are checked, but [foo](foo.md) will just be a broken link.

Should we issue a warning or even throw an error when we encounter a relative non-@ref link to a .md file?