Clarify Apache setup proc in 'Using a Proxy Server [for] REST API'

Edit content to clarify information.

Note: The technical updates are covered in STL-1447:

• Change paths from `/tmp` to a less temporary location.
• For password-file-creation step, explain how the password is hashed.
• For `openssl` step, add a link to `letsencrypt.org`.
• Change Apache and Sawtooth component startup steps to use `systemctl`.