docs: fix links and add link checker

[ayaanoncrypto]

Closes #192

Fixes Issue

The README referenced the retired push-protocol organization and blog location.

Changes proposed

• Updated the blog link to https://push.org/blog.
• • Updated the GitHub organization link to https://github.com/pushchain.
• • Updated the contributing-guide reference to the current repository path.
• • Added .github/workflows/check-links.yml to check Markdown and MDX links on pull requests and pushes to main.
• • Added a short-lived workflow artifact for the link-checker report.

Check List

• My code follows the code style of this project.
• - [x] My change requires changes to the documentation.
• - [x] I have updated the documentation accordingly.
• - [x] This PR does not contain plagiarized content.
• - [x] The title of my pull request is a short description of the requested changes.

Note to reviewers

git diff --check passed. URL replacements were verified with HTTP requests.