Add docusaurus-plugin-copy-page-button

[portdeveloper]

This PR

• adds docusaurus-plugin-copy-page-button to package.json
• adds the plugin string to plugins in docusaurus.config.js
• puts a "Copy page" button in the docs sidebar — exports current page as clean markdown
• adds one-click "Open in Claude / ChatGPT / Gemini" actions that prefill the markdown

why useful for Sequelize specifically:
model definitions, associations, migrations, and the query-interface nuances are very per-page, and the docs are versioned (v6/v7) so the right page matters. the typical flow is reading a docs page → asking an LLM to write the model or query against it. this drops the exact page in as context instead of a hand-pasted snippet. (works across the versioned docs dirs.)

zero config, auto-injects into the TOC sidebar, theme-aware.

shipping on React Native, Redux Toolkit, Puppeteer, pnpm, PlayCanvas, Arbitrum, Cardano, Sui, Nillion, Flare, Kaia, and ~20 other docs sites. listed on the Docusaurus community plugins page.

happy to close/rebase if not a fit

• repo: https://github.com/portdeveloper/docusaurus-plugin-copy-page-button
• npm: https://www.npmjs.com/package/docusaurus-plugin-copy-page-button