Nice plugin! I was demoing it for the first time today and got a 404 the first time I tried to view a handbook page. I quickly guessed this was because rewrites hadn't been flushed and confirmed that by going to Settings > Permalinks, upon which the plugin worked!

I have never had to do this before, but I believe it's possible and user-friendly to flush rewrites on plugin activation to avoid this issue. Again, ⚠I haven't done this before⚠, but register_activation_hook seems to be the place to do this. The docs even include an example.