Added details of abstractions of database availability checks and JSON serialization encoder creation

[AndyButland]

📋 Description

Documents details of an abstraction introduced around boot time checks for database availability, from PR umbraco/Umbraco-CMS#19848 addressing issue umbraco/Umbraco-CMS#17363,

Also details of an abstraction introduced around the creation of an encoder for JSON serialization from umbraco/Umbraco-CMS#19849 addressing issue umbraco/Umbraco-CMS#19812

✅ 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 16.2

Deadline (if relevant)

On 21st August to coincide with the 16.2 release candidate release.

[nikolajlauridsen]

Looks good to me 👍