Docs: update screenshot guidelines

[imatwawana]

Per my conversation with @robbymilo the screenshot guidelines need to be updated as follows:

• docs container is 750 px not 600 px
• Take screenshots on your highest res monitor
• Review screenshots on your highest res monitor
  • Lower res monitors won't give you an accurate sense of what the screenshot looks like on higher res screens
• Screenshots should be twice the size of the container (max width) to accommodate higher res screens
  • Macs already have a 2x dpi so you don't need to do anything to make images twice the size on them, but might need to on a different laptop
  • Pinch and zoom to make a screenshot double the pixel size in cases where that's not already happening