Add section for manual setup of squid in SPO guide

[jpraynaud]

Why

The latest version of Squid is not available from the apt package manager. In order to facilitate the setup process for the SPOs, we need to add explain how to compile Squid manually and setup the associated service.

What

Add a section with compilation process and service setup in the Run a Mithril signer as an SPO guide.

How

• Add a section on how to compile/setup Squid for Mithril
• Add section with ADX check to compile