docs: add SVG version of the Linux logo

In the Rust for Linux patch series, Jonathan Corbet requested [1] that
we avoid binary assets (such as the logo), possibly by recreating
them in SVG format.

However, it turns out there is no SVG version of the Linux logo
in the kernel tree yet, only a GIF one from the pre-v2.0.0 days.

This patch fixes that by adding a SVG version of the Linux logo,
which we could then use in the Rust for Linux documentation and
other similar places.

This is a vector re-illustration by Garrett LeSage of the original
logo by Larry Ewing, plus cleanups by IFo Hancroft.

The file was imported as-is from:

    https://github.com/garrett/Tux/blob/ecd40de64250ea2b24c849e901c3297ad01e54f6/tux.svg

The stated license in the repository is:

> Public domain http://creativecommons.org/publicdomain/zero/1.0/
>
> ...but, if anyone asks:
>
> Tux was originally made by Larry Ewing in the Gimp, re-illustrated
> in vector by Garrett LeSage, using Inkscape and refined and
> cleaned up by IFo Hancro...