fix(v1): versioned_docs and skip-next-release relative path issue

[josephMG]

Motivation

(Write your motivation here.)

Have you read the Contributing Guidelines on pull requests? YES

Due to #1805, reduxjs/react-redux#1583 and reduxjs/react-redux#1588,
I think following codes keep only files in docs/ directory and filter all files in versioned_docs.

https://github.com/facebook/docusaurus/compare/master...josephMG:fix/versioned_docs_relative_link?expand=1#diff-945e90ba000dd59353b89af45929efdeL70

    if (metadata.language !== 'en' || metadata.original_id) {
      return;
    }

After digging into these issues, I notice that metadata.source tracks versionFallback md file path, and htmlLink will be the url of fallback version. So I add a metadata.versioned_source means current version md file path, and htmlLink will be the url of current version.
https://github.com/facebook/docusaurus/compare/master...josephMG:fix/versioned_docs_relative_link?expand=1#diff-229712ff4ab11f33b4d6855bff2cc168L72

Test Plan

Fix Yarn test

Before: (See GIF or https://react-redux.js.org/)

After:

Related PRs

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/facebook/docusaurus, and link to your PR here.)

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[slorber]

LGTM! thanks