CSS-only responsive mobile menus hamburger style

Mostly i cribbed from https://codeconvey.com/responsive-hamburger-menu-css-only/

This is fancier but less relevant: https://moderncss.dev/css-only-accessible-dropdown-navigation-menu/

I don’t think escape to close is so important on a mobile-only visual effect, with a short menu— can just keep moving down to content, we don’t change the way it works really from mobile to desktop to screenreader.