Docs usage: should allow linking with exact path

[rixo]

Currently, ignores ordering prefix and file extension in the URL it generates. So for example 00-foo.md can be accessed at URL foo. This is nice for story like usage, but not so much for docs / markdown preview, where exact path names are often used (e.g. what you'd do in .md targeted at rendering on GH).

Solution: exact paths URL should always resolve to the actual content.