Forms: Add 17 RC release notes and backoffice extension points

[rickbutterfield]

📋 Description

Added information about the 17 RC releases, and information on how to extend the backoffice components using the new @umbraco-forms/backoffice npm package.

The v17 backoffice package is currently ahead of the v16 one. This is why the code examples do not match between versions. The next Forms minor (16.3.0) will match these and the docs will be updated.

📎 Related Issues (if applicable)

N/A

✅ 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)

17.0.0-rc2

Deadline (if relevant)

Today (13/11/2025)

📚 Helpful Resources

• 🧾 Umbraco Contribution Guidelines
• ✍️ Umbraco Documentation Style Guide

[eshanrnh]

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