Since this is a complete rewrite, the diff is sadly not useful for reviewing changes.
This PR also drops the example files for Rust packages, which are very outdated and stale.

A rendered version (minus some fixups that I did after converting from markdown to AsciiDoc) can be viewed here:
https://hackmd.io/@decathorpe/FedoraRustPackagingGuidelines

For previewing AsciiDoc, I find this Firefox addon very useful:
https://addons.mozilla.org/en-GB/firefox/addon/asciidoctorjs-live-preview/

Any feedback and suggestions for improvements are welcome. I wanted to restructure of the Guidelines to address some new use cases that the old Guidelines did not cover at all. But I'm sure that some sections could be expanded / dropped / moved around :)

The text I submitted with this PR has already gone through some small revisions after getting feedback from other Rust SIG members.

Note that some things that were previously documented here have been moved to actual documentation for rust2rpm / cargo2rpm, and some others will be moved to either the Updates Policy or a "Rust Package Maintenance Guide" I'm working on. (And still, this rewrite is about twice the amount of text compared to the old version.)