Clarify BuildKit daemon lifecycle and terminal requirements on Windows

[ngebremariam-msft]

Description

Added a warning to keep the buildkitd.exe terminal open during subsequent steps, and a FAQ section addressing common daemon lifecycle questions for first-time users.

When looking at the BuildKit on Windows documentation, I encountered several areas where additional clarification would improve the user experience and reduce confusion for first-time users.

1) Importance of BuildKit daemon in Step 7

The guide instructs users to start buildkitd.exe and then open another terminal to run subsequent BuildKit commands. However, it does not clearly explain that the terminal running buildkitd.exe must remain open while executing those commands.
Users may close the original terminal, causing the BuildKit daemon to stop unexpectedly. As a result, subsequent BuildKit commands (such as buildctl and docker buildx commands) may fail because they can no longer connect to the daemon.

2) No explanation of daemon lifecycle

After completing a build, users may not know:

• Whether the BuildKit daemon must continue running.
• Whether the image can be run independently of BuildKit.
• Whether the daemon can safely be stopped after the build completes." Does the commit message and description help justify the reason for request of change? If not, create suggestions and explain.

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