docs: add docusaurus-plugin-copy-page-button

[portdeveloper]

Summary

• add docusaurus-plugin-copy-page-button to the docs dependencies
• register the plugin in docusaurus.config.ts

Why

Ceramic docs include developer guides and reference content that are useful to bring into AI tools. This adds a page-level copy/open button that exports the current page as clean markdown and includes one-click Open in ChatGPT, Claude, and Gemini actions.

I checked the repo first for existing page-level copy/Open-in-AI UI and LLM export features (CopyPageButton, DocActionsDropdown, copy as markdown, Open in ChatGPT, Open in Claude, llms.txt, docusaurus-plugin-llms, and similar terms) and did not find an overlapping feature.

The plugin has no runtime dependencies and uses peer dependencies for Docusaurus and React.

Validation

• git diff --check
• package manifest, lockfile, and config sanity check

Not run locally: full docs build, due the dependency install/build footprint in this environment.