Create a new branch named improve-readme in the repository gipsy777/markdoc and prepare a pull request to update documentation and sync with upstream. Do the following steps exactly:

1. Ensure branch improve-readme is created from the current main branch.
2. Add upstream remote https://github.com/markdoc/markdoc and fetch latest main from upstream. Merge upstream/main into the new branch so the branch contains the latest changes from the original project.
3. Update the repository README.md with a concise, clear description. Replace or add a top-level README content with the following (use this as the new README content):

---

markdoc (fork)

This repository is a fork of the original Markdoc project: https://github.com/markdoc/markdoc

Markdoc is a powerful, flexible, Markdown-based authoring framework originally developed by Stripe. This fork currently contains no additional code changes and preserves the original project's MIT license.

If you plan to contribute or customize this project, please sync with the upstream repository, open issues, and create pull requests describing your changes.

Original project: https://github.com/markdoc/markdoc
License: MIT

4. Commit the README change with a commit message: "docs: update README to note fork and upstream link".
5. Push the improve-readme branch to the origin repository (gipsy777/markdoc).
6. Open a Pull Request from improve-readme into main on gipsy777/markdoc with the title: "chore: update README and sync with upstream" and the description below exactly:

---

This PR does the following:

• Creates a branch improve-readme and synchronizes it with the upstream repository markdoc/markdoc main branch.
• Updates README.md to clearly indicate this repository is a fork and includes a link to the original project.

No functional code changes are included. This is a documentation/sync PR to keep the fork up-to-date and clarify repository origin.

If you want additional changes (CI, code modifications), please reply with specific instructions.

7. Do not add CI files or other changes unless explicitly requested. Only perform the steps above.

Provide the created PR URL and a short summary of what was done in the PR after creating it.