feat(content-docs): support custom versioned docs path

[waqarilyas]

Fixes #8061.

This adds a versionedDocsPath option to @docusaurus/plugin-content-docs, allowing projects to keep generated versioned docs and sidebars outside the site directory.

By default, nothing changes: versioned docs are still written to the site directory under versioned_docs and versioned_sidebars.

Motivation

Some projects keep their docs content outside the Docusaurus site root, but docusaurus docs:version always writes generated version folders back into the site directory. This makes monorepo and multi-docs setups harder to organize.

With versionedDocsPath, the docs plugin can write versioned_docs and versioned_sidebars under a configured directory instead.

Changes

• Adds versionedDocsPath to the docs plugin options and TypeScript types.
• Updates version metadata path resolution to read versioned docs/sidebars from the configured path.
• Updates the docs:version CLI flow to write generated docs/sidebars to that path.
• Documents the new option in the plugin API docs and versioning guide.
• Adds regression coverage for the new path helpers and CLI behavior.

Test plan

• pnpm install --frozen-lockfile
• pnpm test packages/docusaurus-plugin-content-docs
• pnpm test packages/docusaurus-plugin-content-docs/src/__tests__/versionedDocsPath.test.ts packages/docusaurus-plugin-content-docs/src/versions/__tests__/files.test.ts
• pnpm eslint --cache --report-unused-disable-directives packages/docusaurus-plugin-content-docs/src/cli.ts packages/docusaurus-plugin-content-docs/src/options.ts packages/docusaurus-plugin-content-docs/src/plugin-content-docs.d.ts packages/docusaurus-plugin-content-docs/src/versions/files.ts packages/docusaurus-plugin-content-docs/src/__tests__/versionedDocsPath.test.ts packages/docusaurus-plugin-content-docs/src/versions/__tests__/files.test.ts
• pnpm oxfmt packages/docusaurus-plugin-content-docs/src/cli.ts packages/docusaurus-plugin-content-docs/src/options.ts packages/docusaurus-plugin-content-docs/src/plugin-content-docs.d.ts packages/docusaurus-plugin-content-docs/src/versions/files.ts packages/docusaurus-plugin-content-docs/src/__tests__/versionedDocsPath.test.ts packages/docusaurus-plugin-content-docs/src/versions/__tests__/files.test.ts website/docs/api/plugins/plugin-content-docs.mdx website/docs/guides/docs/versioning.mdx

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