That’s the post Michael mentioned, in particular, use this comment in the thread:
it worked for me.
Basically, you put static pages into /content/, add front matter and state layout. Then create the actual layouts and they will connect to static content pages, giving you all the power of templates, within static HTML’s.
The unintuitive part is how exactly to set up the front matter on static HTML’s in /content/, and how to prepare the custom layout files for them. Happy hacking!