[New Environment Setup] Add SSL setup instructions

[esoergel]

There are existing instructions for setting up SSL, but they can be simplified if the site is initially setup with a proper domain name (PR here), and if letsencrypt_cchq_ssl: yes is set in proxy.yml.

The remaining steps from ssl.md are quite minimal and can be duplicated in-line in new_environment.md.

This new workflow will of course need to be tested.

https://dimagi-dev.atlassian.net/browse/SC-304