docs: document copy option

[ColumbusLabs]

Description

Add dedicated English and Chinese documentation for the copy option,
including:

• CLI and config-file usage
• paths, globs, and negated patterns
• from, to, flatten, rename, and verbose
• synchronous and asynchronous config callbacks
• build, multi-format, and watch behavior

The pages are added to the localized Options sidebar.

Linked Issues

Implements the unchecked copy documentation item in #247.

Validation

• pnpm docs:build
• pnpm lint
• pnpm test src/features/copy.test.ts --run (6 passed)
• Prettier check
• git diff --check origin/main...HEAD
• rendered-route and localized-sidebar probes

pnpm -C docs run typecheck reports the same five pre-existing errors on this
branch and a clean origin/main worktree (one VideoModal.vue mismatch and
four unresolved virtual/style imports).

Additional context

Assisted by OpenAI Codex. The documentation was checked against the current
copy implementation, configuration resolution, watch plugin, CLI reference,
and unit tests. An independent agent review also checked behavioral accuracy
and prompted a clarification of top-level versus format-specific copy behavior.

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commit: fc0e6b4