move project goals to `goals.rust-lang.org`

[lcnr]

see https://rust-lang.zulipchat.com/#narrow/channel/478266-project-goals.2Fmeta/topic/Where.20is.20the.20PG.20documentation.3F/with/615012637

this would be a lot more discoverable than https://rust-lang.github.io/rust-project-goals/

[lcnr]

done