Generate `index.html` and `404.html` programatically

[avivkeller]

Now that we use parallel processing, there's no reason to pass the index via --index. The index array can be generated programatically. To that extend, the entire page can be generated.

Additionally, we can generate 404 pages.

[ovflowd]

There were reasons behind using said file, fyi. In the next redesign of the docs (not the visual redesign, but the structural redesign proposed to next-10 a few years ago nodejs/next-10#166 we wanted index files to be able to be customizable.

[AugustinMauroy]

can we parse/intermet index page differently to have such as card + stability index on this card. So md keep the same structure is just the way that we interpret it

[avivkeller]

blocked by #780, I think, since this'll be a part of that generator