[website] - Rework frontend layout

[Gregofi]

The layout now assumes that the top navbar is fixed size, which if it had not existed would fix all the responsive issues, since on mobile the menu would just grew larger.

This is because the main div has to have fixed height, which is 100vh - size of the navbar. It must have fixed height, because otherwise the stdout/err containers would cause an overflow in the whole page (even though they have overflow of their own).