docs: showcase DocSentinel console workflow

[arthurpanhku]

Summary

• Add a compact README product tour GIF generated from a locally running DocSentinel console.
• Summarize the main surfaces: Command Center, Assessment Workbench, Governance Portal, Knowledge Base, Agent Integrations, and Settings.
• Add a DeepSeek configuration example using a placeholder API key and a reminder not to commit real secrets.

Validation

• Built the frontend and ran the FastAPI app locally to capture the screenshots used in the GIF.
• pre-commit run --all-files -> passed.
• git diff --check -> passed.
• Sensitive-content grep -> zero matches.
• Secret scan for the provided DeepSeek key and high-confidence sk-* patterns -> zero matches.

Notes

The user-provided DeepSeek key was not written to the repository or demo assets. The README uses sk-your-deepseek-key as a placeholder.