Show tree structure on `index.html`

TODO:

- [x] Add a traceability matrix to `index.html`
- [x] Sort sibling documents by prefix (for a consent column order)
- [x] Add `Tree` method to display the [tree as text][1]
- [x] Add a similar structure to generated `index.html`
- [x] Handle unknown items in the traceability table

[1]: http://www.linemanjs.com/#directory-structure