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>

README.rst

@@ -2,7 +2,7 @@
 
    <a href="https://www.zephyrproject.org">
      <p align="center">
-       <img src="doc/_static/images/logo-readme.png">
+       <img src="doc/_static/images/logo-readme.svg">
      </p>
    </a>