README: Update & Small improvements

[tooomm]

• Change links to use new repo name
• Generalized wording, yet clarified when something was specific to a model
• Use markdown alerts to highlight some sections
• Other small fixes for e.g. code indentation or comments for readability

Check the result in my repo: https://github.com/tooomm/ugreen_leds_controller/tree/rename

After the repo rename reflecting the broad device support, I suggest you also update the repo description: It could also read An LED Controller for UGREEN's DX/DXP NAS Series