chore: set up @icp-sdk/vetkeys docs in icp-js-sdk-docs

[marc0olo]

Summary

Set up proper documentation for @icp-sdk/vetkeys integrated with the centralized dfinity/icp-js-sdk-docs system, following the icp-js-auth docs pattern.

Tasks

• Create a docs/ subpackage (own package.json, pnpm) with @dfinity/typedoc-plugin-icp-docs, typedoc-plugin-markdown, typedoc-plugin-frontmatter
• Write initial content: installation, quick-start, API reference
• Replace .github/workflows/publish-frontend-docs.yml with the icp-js-auth publish-docs.yml pattern (push to dfinity/icp-js-sdk-docs)
• Hook docs publish into release.yml (triggered on non-beta releases)

Blockers

• Requires understanding @dfinity/typedoc-plugin-icp-docs availability and the dfinity/icp-js-sdk-docs repo setup
• Depends on chore: migrate @dfinity/vetkeys to @icp-sdk/vetkeys #382 (package migration) being completed first