Update guidelines and wording around menu banners

[Walavouchey]

the website now displays them on the front page, which means designers need to consider how they appear there too. they generally need transparent padding above and below the image and look off if they're designed to stick to the bottom of the screen

"menu banner" may also be too specific (i notice i've used "menu/website banner" recently). "system title" is their internal/code name, but it might be more confusing than descriptive, idk