docs: update Digital Ocean installation instructions

Created by: beyang

Partially address https://github.com/sourcegraph/sourcegraph/issues/6652. Previously, the instructions did not yield a functioning instance.

This also changes docs to server HTTPS on port 433 (standard) rather than 80 (non-standard).

What remains to be done for #6652 (closed) is to test out the path for enabling IPv6.