Update and restructuring of README.md

[Itangalo]

[Itangalo]

Some notes on the changes for the readme:

• I put screenshots of the interface at the top. My assessment is that this is what people come for, and will stay for. (Twitter and link to diffusionbee.com put after these images actually to improve their visibility – they were drowned when displayed above.) Also, new screenshots were created to reflect updated interface.
• On "why use DiffusionBee" a new item was added mentioning the speed.
• Requirements were updated. (Messages on Discord indicated that MacOS 12.3 or later works.)
• Link to documentation instead of having docs in the readme.
• The previous reference 1 (orphaned?) is kept but with a different description. Could be worth checking.