Add Display to mdbook clean

[dcampbell24]

Problem

There currently is nothing displayed when I run mdbook clean. Display something similar to cargo clean.

I didn't realize I was supposed to open an issue first, but this is a small change with a real impact.

Proposed Solution

#2458

Notes

No response

[dcampbell24]

@ehuss In CONTRIBUTING.md it says please ping us before working on a pull request. It is not clear who us refers to. I am pinging you as you have a lot of recent merges. Could you clarify who us is in the CONTRIBUTING.md?

[szabgab]

The related PR was accepted. Please close this issue.