Add initial MkDocs documentation site

[0russwest0]

Summary

This PR adds an initial documentation site for Agent-R1 based on MkDocs. It introduces a lightweight but structured reading path covering setup, core concepts, and the main multi-step agent workflow.

Changes

• add MkDocs site configuration in mkdocs.yml
• add documentation entry pages for Home, Getting Started, Core Concepts, and Tutorials
• add a minimal setup path with installation and quick start guidance
• add core concept documentation for Step-level MDP and Layered Abstractions
• add an Agent Task tutorial focused on AgentEnvLoop and ToolEnv
• update README.md to point users to the new documentation flow
• add docs/requirements.txt for documentation dependencies
• add a GitHub Actions workflow to deploy docs

Notes

• this is an initial lightweight documentation pass focused on the current core Agent-R1 workflow
• the docs intentionally prioritize onboarding and conceptual clarity over exhaustive API coverage

[Melmaphother]

Nice work

[Melmaphother]

No problem