Update content for `level@8` and `abstract-level`

[vweevers]

Follow-up for #62. There are some links to update, and the introduction too.

[vweevers]

Trying out some new colors, and making it more playful:

leveljs.colors.mp4

[vweevers]

[vweevers]

Homepage is 90% done (I need to convert the logo to an SVG, make the layout responsive, tweak a few colors, and remove some Lorem Ipsum content - like "FreeBSD" which we do still support but I don't want to commit to that):