Include non markdown files in navigation

[jroper]

You may have some html files that you want included in tocs and navigation that are not generated from markdown - for example API docs (in my case, I have multiple embedded api docs, for different languages that this project supports, and I want them to be part of the overall navigation). It would be ideal if these could be included in the navigation, eg:

@@@index

* [Foo](foo.md)
* [Bar](bar.md)
* [API docs](api/index.html)

@@@

[ennru]

Kind of duplicate of #286. As groundwork I introduced the @link directive, but it is not allowed in @@@index, yet.