Include the canonical path to the current document when rendering a document.

Given a document in WIKI_ROOT/foo/bar.md, it's tricky to reconsruct "foo/bar"
or "foo/bar.html" or "foo/bar.md" from the breadcrumbs. A similar bit of
context (named "directory") is already passed to directory templates; now
markdoc passes the document path to the document template too.

I'm using this in my own markdoc instance to generate "page history" links (to
github) for each page.