docs: expand package READMEs with usage, API, and examples

[lukascivil]

Objective

The package-level READMEs were either missing or one-liner stubs, leaving npm visitors and new contributors without enough context to install, run, or wire each package into a project. This rewrites them so each package is self-explanatory from its own README.

What was done

• Added a brand-new README for libs/json-difference covering install, usage, full API reference (getDiff, getStructPaths, getEditedPaths, getPathsDiff), the Delta shape, options, path format, a reference-operations table, and CDN usage
• Added a brand-new README for libs/json-difference-cli with install, jd -o … -m … usage, options table, examples, shell-quoting tips for macOS/Linux/PowerShell, and output format
• Expanded tools/mcp-json-diff/README.md: split capabilities into Tools vs. Prompts sections, added Why/Requirements, Claude Desktop and Claude Code setup (with config paths per OS), inspector instructions, troubleshooting table, log locations, and a Development section
• Rewrote apps/example/README.md to document src/simple.ts and src/stress.ts, how to run via nx or tsx directly, expected output, and how to add a new case
• Rewrote apps/playground/README.md with the live demo link, stack (React 19, Chakra UI v3, Monaco, Vite), local dev / build / preview / lint / type-check commands, and a deployment note pointing at the CD workflow
• All READMEs cross-link related packages (lib ↔ cli ↔ mcp ↔ playground) and reference the repo, issues, and changelog where applicable

Test plan

• Render each modified README on GitHub and confirm tables, code blocks, and relative links resolve
• Verify the npm-published packages (json-difference, json-difference-cli) will display the new READMEs correctly (no broken relative links to monorepo paths that don't ship in the npm tarball)
• Confirm the live playground link and MCP config snippets are accurate