Rerwite the "Additional preview environments support" article and correct URL provider references

[kjac]

📋 Description

This PR documents how to setup external preview URLs for Umbraco 17+.

It also amends all mentions of IUrlProvider with the additions made to the interface in Umbraco 17.

📎 Related Issues (if applicable)

None

✅ 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)

V17

Deadline (if relevant)

Whenever possible.

📚 Helpful Resources

• 🧾 Umbraco Contribution Guidelines
• ✍️ Umbraco Documentation Style Guide

[eshanrnh]

Thanks for the PR, @kjac 🙌 The PR looks great 💪

I have added a few suggestions to include prepositions, add or remove commas, and ensure style and tone consistency as per the style guide. I will take care of those.