Docs: Improve and expand on webhook signature testing

[bee-anvith]

Description

This PR cleans up and expands the documentation on webhook signature testing to recommend better, more accessible tools.
Key Changes:

• Expanded Tooling: Added a segment for local testing/inspection tools and cloud-based webhook testing services.
• New Recommendations: Replaced RequestBin with Webhook.site and Beeceptor for instant, zero-setup testing endpoints. RequestBin requires account creation, which adds multiple steps.
• Clarity: Improved grammar to be more descriptive and professional.

That aside, I noticed that the link on step 2 [Configure](#configure) points to an anchor URL that doesn't exist. I have left it untouched.

Checklist

• Commits have been rebased.
• Linting has been run against the changed file(s).
• The PR adheres to the writing style guide and contribution guide.

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on this repository.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands and usage tips.}