add links and remove broken from implementation specific chat section

[spdegabrielle]

1. removed broken implementation links that currently 404
2. add links to Racket chat services as single section
3. change broken guile and loko links (e.g. https://get.scheme.org/guile/) to guile.scheme.org and loko.scheme.org respectively

[arthurgleckler]

Thanks. Merged and pushed.