docs: Restructure quick build section to clarify runqemu

Standard and OpenHarmony-compatible images require different documentation for running a qemu-based system. Restructure quick build document to clarify this.

Signed-off-by: Andrei Gherzan andrei.gherzan@huawei.com

requested review from @fldn, @kristis, @idlethread, @robertd, and @thierrye

assigned to @agherzan

mentioned in merge request !207 (merged)

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merged