Refactor the CSS-only solution for collapsed TOC once grid is merged. We want to maintain the existing absolute positioning based solution for grade C browsers
Acceptance criteria
- TOC can collapse into the page title on using grid
- TOC at narrow screens on grade C browers is usable
- no UI regressions
Developer notes
Although the current solution works with grid enabled, we want to update the CSS to work on larger screen sizes using grid, in order to unblock follow up work (T307901)