docs: Add system architecture and user flow diagrams

[Vladush]

Description

Adds comprehensive visual documentation for the system architecture and user authentication flows. New standalone docs (ARCHITECTURE.md, USER_FLOWS.md) use Mermaid diagrams to illustrate component relationships, the PAM authentication pipeline, and enrollment/authentication sequences. Existing guides (README.md, CONTRIBUTING.md, CONFIGURATION.md) are updated with cross-references to the new pages.

Type of change

• Documentation update

How Has This Been Tested?

• Verified Mermaid diagrams render correctly on GitHub
• Confirmed all cross-reference links between docs resolve properly
• Reviewed for consistency with existing documentation style

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes