Wide Mermaid charts overflow

Previously, overly wide Mermaid charts would be resized to fit within the middle column of the docs site.

At some point, this behavior changed such that the charts now render better (aren't resized, possibly rendering them unreadable), but overflow the available viewing area.

For example: https://docs.gitlab.com/ee/development/architecture.html#component-diagram.

An MVC would be to keep the extra width (allowing horizontal scrolling to view components), but layering the ToC so that it obscures the chart rather than the chart appearing within it.