imporved the check-broken-link-md.py script according to the description

[Aaqilyousuf]

Description:

This PR addresses and resolves#222 by improving the check-broken-links-md.py script to make it more robust, maintainable, and reliable.

Changes Made:

• ✅ Fixed whitelist handling so only non-exempt URLs are flagged.

• ✅ Improved subprocess handling to separate stdout/stderr for clearer debugging.

• ✅ Replaced os._exit(1) with sys.exit(1) for safe process termination.

• ✅ Introduced regex-based parsing instead of fragile string checks.

• ✅ Refactored code into smaller, testable functions.

• ✅ Centralized constants (e.g., 429 status code, exception list).

• ✅ Added support for configurable exception lists (via CLI or JSON/YAML).

• ✅ Enhanced output with clearer summary of links checked, failed, and skipped.

Why this is needed:

These changes make the script easier to maintain, extend, and debug, while ensuring accurate link checking results and safer process handling.

Screenshots

[Aaqilyousuf]

@xmnlab Changes are done

[xmnlab]

@Aaqilyousuf could you rebase this pr on top of the upstream main pls?

[xmnlab]

@Aaqilyousuf , I am planing to review your PR tomorrow. thanks!