feat: mobile docs toc button

[EisenbergEffect]

When the docs are displayed on mobile, the ToC collapses and a button appears in the bottom right corner of the screen, which can be used to display the ToC. The style and location of the button make it hard to discover. Let's see if we can at least style the button with an accent color as a minimum. If we can also rework the CSS to position it in the top right, that would be even better.