More specific instructions for creating Proxy Authentication token

[camtauxe]

Overview

The instructions for creating the HMAC token (recommended for proxy authentication) aren't specific enough for developers to actually create a token from a username and secret. This PR clarifies the instructions, adds an example (using openssl) of creating the token and then adds it to the example HTTP request.

Testing recommendations

• Make sure documentation and syntax highlighting look okay on webpage
• Check that the example openssl command works and produces the same HMAC

Checklist

• Update rebar.config.script with the commit hash once this PR is rebased and merged

[wohali]

Thanks for your contribution, @camtauxe !