Add initial private state manager docs

[MCarlomagno]

This PR introduces a new section in the documentation about Private State Manager, including:

• Index: Introduction to the system, why does it exist.
• Overview: How it works what are the main flows and components.
• Multisig: Introduces how it helps to run private multisigs along with some links to SDKs.
• API Reference: Details the PSM APIs and the client wrappers (rust and typescript).
• Operator guide: How to setup and run a server.

This is just an initial documentation surfacing the main concepts and ideas, over time we can add more details, diagrams, tips and examples