The Guide to the Rust Runtime needs to be updated

[aochagavia]

As a consequence of #17325 (removing of libgreen), the Guide to the Rust Runtime needs to be updated. Currently, there are many references to libgreen thar aren't valid anymore.

[aochagavia]

cc @steveklabnik

[thestinger]

It needs removal, not a rewrite. None of the information is going to be relevant once the work is complete and it's not accurate now.

[aturon]

Yes, this guide needs to be dropped completely.