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 Capacitor specifically:
the plugin API and iOS/Android setup pages carry a lot of per-page nuance — permissions, native config, platform quirks. the typical flow for an app dev is reading an API or guide page → wanting to drop just that page into an LLM to scaffold the integration or debug a native build issue. this saves the select-and-clean step and keeps the model grounded in the current API rather than a stale training snapshot.

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

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