The normative change here is to resolve "TODO: should the event even fire at all, for these latter two cases?" in the negative: i.e., the navigate event will not fire at all for non-interceptable cases. Additionally, this makes same-document fragment navigations, even initiated via browser UI or window.open(), interceptable.

Beyond that, this commit:

* Expands the discussion of navigation interception to be clearer about what "user-initiated" means
* Adds an appendix detailing all the navigations on the platform, the distinction between cross-document and same-document (closes #3), and a summary table listing how this spec works for each type.
* Discusses the connection between the navigations we're considering, and the HTML Standard algorithms.