docs: Clean public docs surface

[rahulbsw]

Summary

• replace the stale .github/README.md with a short maintainer-focused note that points public readers to the real project docs
• rewrite docs/USAGE.md around selective replication, shaping, redaction, and routing instead of the older generic streaming examples
• tighten docs/_config.yml so GitHub Pages excludes archive, benchmark, superpowers, and internal engineering docs from the main public docs surface

Verification

• git diff --check
• ruby -e 'require "yaml"; YAML.load_file("docs/_config.yml"); puts "ok"'

Notes

• leaves local untracked folders such as graphify-out/ untouched
• focuses on public docs surface cleanup after PR docs: Finalize selective replication launch surface #18