ci: add GitHub Action to enforce markdown style with pymarkdown

[mattheliu]

🧩 What does this PR do?

This PR introduces an automated Markdown quality gate for the ERNIE repository by adding a GitHub Actions workflow that runs pymarkdown on every push and pull request affecting Markdown files.

It ensures that all documentation changes comply with the project's formatting standards before being merged.

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🚀 Why is this needed?

Documentation quality is critical for usability and long-term maintenance.
Before this PR, Markdown style checks were performed manually, which was error-prone and inconsistent.

This workflow provides:

• Automatic enforcement of Markdown conventions
• Early detection of formatting issues in PRs
• Reduced review overhead for maintainers
• Improved consistency across documentation

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🛠 What was added?

• New GitHub Actions workflow:
  .github/workflows/markdown-lint.yml
• Integrated pymarkdown as the Markdown linting engine
• Triggered on push and pull_request for all .md files

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🧪 How is it tested?

The same command is used both locally and in CI:

pymarkdown scan source/

[paddle-bot]

Thanks for your contribution!