Creating documentation about Windows helper images

What does this MR do?

Creating and modifing the documentation about Windows helper images.

Why was this MR needed?

The documentation lacks clarity regarding how Windows helper images are selected and used, specifically in two areas:

1. Windows Server 2025 helper image: There is no documentation explaining that Windows Server 2025 hosts use the Windows Server 2022 (ltsc2022) helper image. Users might look for a specific 2025 helper tag and assume support is missing. The docs should explain that this works by design due to Windows down-level compatibility (as implemented in !5170 (merged)).
2. Nano Server vs. Server Core helpers: The documentation does not clearly explain the difference between the nanoserver and servercore helper variants. There is no guidance on why both exist, which one should I use and why, or how the Runner determines which one to pull.

The documentation should be updated to clarify how the Runner chooses the appropriate helper image based on the host OS, explain the purpose of the Nano/Core variants, and explicitly document the down-level compatibility fallback for newer versions like 2025.

What are the relevant issue numbers?

#39326 (closed)