Add Forms 16 RC release notes and update version specific upgrade notes

[ronaldbarendse]

📋 Description

Add Forms 16.0.0-rc1 and 16.0.0-rc2 release notes and update version specific upgrade notes.

📎 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)

Forms 16

Deadline (if relevant)

Direct: Forms 16.0.0-rc1 was released May 15th and 16.0.0-rc2 on the 26th.

📚 Helpful Resources

• 🧾 Umbraco Contribution Guidelines
• ✍️ Umbraco Documentation Style Guide

[eshanrnh]

Looks good to me, @ronaldbarendse 🙌