docs: Adding original source of the metal_stack diagram.

Source file can be used with
[xcircuit](https://github.com/RTimothyEdwards/xcircuit).

Export to SVG from xcircuit using menu item "File->Export->Export SVG"
(from the first page).

Signed-off-by: Tim 'mithro' Ansell <me@mith.ro>