readme: make README logo dark-mode aware

Use an SVG based logo with embedded style that toggles title color using
media queries. The SVG logo is not exactly the same as the previous one
as I only had this version in SVG (taken from docs actually).

Note that we cannot use the method described in
https://github.blog/changelog/


2021-11-24-specify-theme-context-for-images-in-markdown/ because the
README is written in rst, not Markdown.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>