Enhance Checkfront Webhooks Documentation with Examples & Testing Tools

[ssganesh035]

1. Clarified how webhooks work, including event triggers and retry mechanisms.
2. Added a table outlining supported webhook data formats.
3. Included an example JSON payload for better understanding.
4. Provided a Flask-based webhook handler for quick implementation.
5. Ensured proper reStructuredText (.rst) formatting for GitHub compatibility.

[ssganesh035]

Just checking, if this is a good change to improve docs. Let me know if there is any change needed.