Use PNG instead of SVG as Gitiles doesn't render SVG well.
In order to make ImageMagick's conversion work well, fill-opacity and
stroke-opacity are explicitly specified in the SVG.
Also, the PNG image will be put at the bottom of README so people can
read contents at first glance without scrolling over the logo.
BUG=none
TEST=check gitiles view on Gerrit
TEST=mdbook build
Change-Id: I2203be1258265400be3caf2ad8d8a1e8cacc5aab
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3250151
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Dennis Kempin <denniskempin@google.com>
Commit-Queue: Keiichi Watanabe <keiichiw@chromium.org>
