[docs] Remove MarkdownElement import

[Janpot]

Fixes #3442

Also fixes the scroll bar under the tab container, and clips the rounded corners correctly

Remove MarkdownElement import from src/components/markdown/MarkdownElement
See mui/material-ui#41859 (comment)

Aside, HighlightedCode is a bit hard to use imo.

Intuitively I'd expect three levels of abstraction here, so that I can mix and match to my use case.

1. a highlighted code block, which auto sizes with the height, no border/corners
2. a code viewport, which provides the scroll behavior
3. a container which provides title/tabs and clips the corners

Preview: https://deploy-preview-3451--mui-toolpad-docs.netlify.app/toolpad/

before:

after: