See @mbostock’s post for a better way to get a runnable version of your notebook! 
Perhaps I’m misunderstanding your question, but if you know how to embed selected cells, then you can embed your entire notebook by deciding to embed every cell.
More explicitly, the easiest way I know of to embed selected cells from a notebook into other (non-Observable notebook) web pages is to export your entire notebook (à la the “Downloading and embedding notebooks” tutorial) and write a script using the notebook-runtime module to render the specific cells you want.
If what you’re asking is how to embed the Observable website’s exact interface to your notebook (including editing, etc.), then unfortunately, that’s not possible (yet?); at least not without reverse-engineering a lot of the interface / look-and-feel on your own, as well as doing some amount of parsing and processing of your exported notebook source.