Add background line CSS animation to Rill dev welcome page

[jkhwu]

Reference for generally what the animation should look like:

Rill dev welcome page

https://www.figma.com/file/Qt6EyotCBS3V6O31jVhMQ7/RILL?type=design&node-id=1987-35610&mode=design&t=Quuxx63EyXl427U9-11

• See here for separated layers: https://www.figma.com/file/Qt6EyotCBS3V6O31jVhMQ7/RILL?type=design&node-id=8372-832485&mode=design&t=2SlPTPxwRkk2Srpl-11
• See here for starting point on line animation: https://codepen.io/jkhwu/pen/NWJvNqo

[mindspank]

@briangregoryholmes a fun one to tinker with when you get some time :)