docs: add CLI reference and project overview, and update README links

[tom-nash]

Motivation

• Improve discoverability and onboarding by providing a dedicated CLI reference and a high-level project overview.
• Surface those documents from the main README.md so contributors and users can find detailed usage and concepts quickly.

Description

• Add a new docs/cli-reference.md containing command usage, flags, examples, and notes for the cordon CLI.
• Add a new docs/project-overview-and-concepts.md describing the project's goals, architecture, and contributor reading order.
• Update README.md to include a Documentation section that links to docs/cli-reference.md and docs/project-overview-and-concepts.md.

Testing

• This is a documentation-only change so no automated tests were required for the modifications.

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Codex Task