Swap from superlinter to a markdown-specific linter

## What problem does this solve or what need does it fill?

superlinter handles a ton of different languages. We don't need that, and swapping to a simpler, purpose-built tool would reduce churn (e.g. #18320) and may improve performance.

## What solution would you like?

https://github.com/akiomik/mado is written in Rust (so we can easily help maintain it), and is advertised as being much faster.

## What alternative(s) have you considered?

Find some other markdown linter.

#8471 tried this with `markdownlint-cli2-action`, which stalled out.

## Additional context

The `mado` author's [Github profile](https://github.com/akiomik) lists them as a bird lover.