Expand Maskrom download mode documentation

Nicolas Frattarolirequested to merge
fratti/notes-for-rockchip-3588:fratti/maskrom-docs into main

Add a description of the procedure to enter Maskrom download mode for the ROCK 5B+, and add explanatory diagrams for both ROCK 5B and ROCK 5B+.

Also reword the paragraph slightly for readability.

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