docs: move Buildroot CSS code into its own file

In preparation to the addition of more CSS code, this commit moves the
Buildroot site specific CSS code from header.html to a dedicated CSS
file.

[Thomas: add missing space before opening curly brace, as suggested by
Thomas DS.]

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>