Sign providers and backoffice signs

[AndyButland]

📋 Description

This PR adds details of sign providers and backoffice signs that are targeted for 16.3. It will be added to with the client-side details when they are ready.

📎 Related Issues (if applicable)

Relates to PR: umbraco/Umbraco-CMS#19806

✅ 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)

CMS 16.3

Deadline (if relevant)

Expect this to be published along with the RC for 16.3 which is scheduled for 2nd October.