doc/nrd: add example of multipage rendering

  ]

}

```

## Rendering multiple pages

The `include` directive accepts an argument `into-file` to specify the file into which the imported markdown should be rendered to. We can use this argument to set up multipage rendering of the manuals.

For example

~~~

```{=include=} appendix html:into-file=//release-notes.html

release-notes/release-notes.md

```

~~~

will render the release notes into a `release-notes.html` file, instead of making it a section within the default `index.html`.