Remove outdated "Deploying & using a mirror" instructions

[Turbo87]

This document is five years old, and while some of the information is still correct we do not support running the crates.io project as a registry mirror (anymore). We will remove this piece of documentation here to avoid confusion for any users looking at this.

[CapableWeb]

Would it maybe be more appropriate to point users in a different direction if it's no longer advised to run things that way?

As it stands right now, bunch of other things point to the MIRROR.md document and as this is the internet, there is bound to be lots of links that now leads you to a unhelpful 404 page instead of just a page describing the old process + message about it has been deprecated.

As an example, searching for "cargo mirror" on Google shows the MIRROR.md document as the first hit, but that link now returns 404. Instead, it would be great if the existing information remains, but a notice is added to the top of the document referring the user elsewhere.

FWIW, I just happened to discover this as I was in the process of setting up my own local mirror and refreshed the browser page as I came back after launch, and discovered it now returned 404...

[Turbo87]

that's a fair point. I don't know what else to recommend as alternatives though. all I know is that crates.io hasn't officially supported mirroring for multiple years at this point.