Live/ready checks and unattended upgrades

[AndyButland]

📋 Description

This PR adds details of live and ready health check probes, the UpgradingViewPath configuration setting and updates to the boot and HTTP responsiveness behaviour in unnattended upgrades.

📎 Related Issues (if applicable)

umbraco/Umbraco-CMS#22020

✅ 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 17.3

Deadline (if relevant)

Should be published along with the release of the RC for 17.3, which is scheduled for 19th March.