Documentation for `ref` and `relref`.

- Rejigged the weight of the extras/ content for the new crossreferences
  page.
- Used the new {{%/*…*/%}} format for documenting highlighting and to
  prevent a warning about the missing `fig` shortcode.
Change file unique ID to relpath.

- This ensures that `post.md` and `blog/post.id` have *different*
  document IDs.