Dgeni is now providing the `id` for all the documents to be viewed. So we
no longer need to add this to the DocumentContents object.
There are some notable changes in the refactoring:
`DocumentService`:
* The id of the document to render is now obtained from `LocationService.path()`.
* The `getFileNotFoundDoc` and `getErrorDoc` methods have been extracted from
the `fetchDocument` method.
`AppComponent`:
* the `pageId` is now computed in a separate `setPageId` method.
`AppComponen` spec file:
* The `TestHttp` has had the hard coded documents removed and replaced with
a function that will generate docs as needed.
Previously, the `AppComponent.pageId` was set via the current URL, rather than
the document being displayed. This is only really noticeable when the URL does not
match a valid doc and we are actually displaying a 404 page.
Now we compute the `pageId` from the URL of the document being viewed,
which is returned from the `DocumentService.currentDocument` observable instead.
Content pages like `tutorial/index.md` were being mapped to `tutorial.index.json`,
which meant that they could only be rendered if you browsed to `/tutorial/index`.
This didn't sit well so now these pages are mapped to `tutorial.json`, which
means that you browser to them via `/tutorial/` or just `/tutorial`.
Fixed#15335
