docs: rewrite usage documentation

[hughgrigg]

Rewrites the README and every topic guide around the current Rainlytics API. It adds a complete getting-started path from CDK installation through the first stored report. It also replaces design-history-heavy explanations with direct setup and usage guidance.

• Conventional commit message, used as the title
• Conventional branch name, like feat/concise-description
• Full check with pnpm run check passed
• Rebased off latest main
• User-facing behaviour is documented in docs/

The documentation, formatting, linting, build, package and beacon-size stages pass locally. The Yulin-backed coverage stage loses simulator state on this host, including from a clean worktree under Node 26. CI will rerun it.

[coderabbitai]

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