Add automated Markdown linting and link checking to improve documentation quality

[Nomzy-kush]

Fixes #2005

I added automated docs-quality checks to address the issue of inconsistent Markdown styling and unnoticed broken links.

What I implemented

1. A GitHub Actions workflow that runs on every PR touching *.md files.

2. Markdown linting using markdownlint-cli.

3. External link validation using lychee.

4. Shared config files (.markdownlint.json and lychee.toml) at the repo root.

5. Updated CONTRIBUTING.md with clear steps for running both checks locally.

Result

The repository now enforces consistent Markdown formatting, catches broken links automatically, and gives contributors the tools to validate their docs before opening a PR.