Add docs about Umbraco Commerce payment/shipping provider settings validation

[umbracotrd]

📋 Description

Add docs about Umbraco Commerce payment/shipping provider settings validation. It's a new feature in Umbraco Commerce 16.4.0

📎 Related Issues (if applicable)

• Add ability for configure `Validation` property throught `PaymentProviderSettingAttribute` Umbraco.Commerce.Issues#546
• Add option to set payment provider settings property as mandatory Umbraco.Commerce.Issues#623

✅ Contributor Checklist

I've followed the Umbraco Documentation Style Guide and can confirm that:

• Code blocks are correctly formatted.
• Sentences are short and clear (preferably under 25 words).
• Passive voice and first-person language (“we”, “I”) are avoided.
• Relevant pages are linked.
• All links work and point to the correct resources.
• Screenshots or diagrams are included if useful.
• Any code examples or instructions have been tested.
• Typos, broken links, and broken images are fixed.

Product & Version (if relevant)

Umbraco Commerce 16.4.0

Deadline (if relevant)

When Umbraco Commerce 16.4.0 is released, which is planned for October 30, 2025.

📚 Helpful Resources

• 🧾 Umbraco Contribution Guidelines
• ✍️ Umbraco Documentation Style Guide

[eshanrnh]

Thanks for the PR, @umbracotrd 🙌 We will review it asap.

[eshanrnh]

The PR looks great, @umbracotrd 🙌 I have added a few suggestions focusing on style guide adherence and readability.

Will merge it on Oct 30.