docs: refresh README landing page

[lan17]

Summary

• Reposition the README around HomeSec as a local-first AI security camera system and self-serve control plane.
• Replace the stale TOC/quickstart-heavy structure with Docker-first setup, UI overview, pipeline architecture, config/security notes, and current integrations.
• Compress lower-level plugin/development content while keeping links to deeper docs.

Validation

• git diff --check
• Custom README anchor/local-link validation script
• Graphify orientation/impact queries for README, UI/setup, config, and docs references

Notes

• Docs-only change; no runtime code changed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.16%. Comparing base (0d677d9) to head (fed8005).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #107   +/-   ##
=======================================
  Coverage   87.16%   87.16%           
=======================================
  Files         151      151           
  Lines       16422    16422           
=======================================
  Hits        14314    14314           
  Misses       2108     2108           

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