Create website 404 page

[vcoisne]

https://github.com/src-d/product/issues/291 for context.

1. use the gitbase playground server connectivity broken robot animation and work around the copy for this page, what do you think?

@ricardobaeta can you point @znegrin to the source file ?

2. Also let's (very briefly) check a few 404s from benchmark companies to derive some best practices there (like guiding the user somewhere besides just saying it is a dead end) when working on this.

[ricardobaeta]

@znegrin Here's the asset for the broken robot animation https://drive.google.com/file/d/1pxISPPkfNyBgZHpKe7NsZz4_hpN_LF0x/view?usp=sharing