[DOCS] Introduce a Security and Threat Model documentation

Description
Currently, SaltStack doesn't have a Security and Treat Model docs. It's unfortunate, especially after so many critical CVEs in the past.

Salt has a tiny security doc here: https://docs.saltproject.io/en/getstarted/system/communication.html
And a hardening guide that is a bit strange without Security and Threat Model docs: https://docs.saltproject.io/en/latest/topics/hardening.html

Suggested Fix
I'd say we need to write a doc that:

• Describe Salt networking communications
• Describe(in detail!) Salt custom AES keys system
• Describe key signing, revocation, rotating
• Describe Salt Auth system for: CLI, API, Engines
• Describe Salt Authz system
• Describe Pillars and Secrets handling
• Describe possible security differences between 0MQ and TCP transports
• Describe possible security implications of using Syndic\MoM.
• Describe the SaltStack threat model

Type of documentation
Security docs

Additional context
A good example of such doc: https://www.nomadproject.io/docs/internals/security

Start by reading Salt's existing communication documentation and hardening guide, then compare their coverage with the Nomad security documentation example. Define the threat model and document networking, AES keys, key lifecycle, authentication, authorization, pillars and secrets, transport differences, and Syndic/MoM implications. Done means a reviewed security and threat model document covering these areas.