Every guide gets a table of contents — and the layout bug that had been bleeding every page off the right edge since forever

The ToC was the goal; the layout fix was the surprise. `mx-auto` on a flex item silently disables stretch, so `<main>` had been sizing to its content instead of its parent on every page of the site. Found by driving headless Chrome after three wrong guesses. Plus a new orientation on which open model does which job.