docs: simplify onboarding and demo explanations

[rlippmann]

What changed

• Simplified the top-level onboarding language in README.md to be behavior-first and easier for new users to scan.
• Clarified wording in core docs:
  • docs/README.md
  • docs/DescriptionAndMilestones.md
• Simplified integration documentation phrasing in:
  • examples/integrations/README.md
  • examples/integrations/openwebui/README.md
  • examples/integrations/litellm/README.md
  • examples/integrations/litellm_proxy/README.md
• Performed a conservative clarity pass on demo-facing docs in demos/README.md (user-facing explanation text only).
• Bumped package version to 0.6.20 in pyproject.toml and the corresponding uv.lock entry.

Why

• Make the project easier to understand for first-time users without changing behavior, contracts, or evidence.
• Replace abstract phrasing with smaller, concrete wording where safe.
• Improve demo readability so users can quickly see what behavior changes they should notice.

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)