readme: Add support for light/dark modes in the logo

Use the mechanism described here:
https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#specifying-the-theme-an-image-is-shown-to

to provide different logos for light and dark mode, so that the "Zephyr"
text is clearly visible in both.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

README.rst

@@ -2,7 +2,11 @@
 
    <a href="https://www.zephyrproject.org">
      <p align="center">
-       <img src="doc/_static/images/logo-readme.svg">
+       <picture>
+         <source media="(prefers-color-scheme: dark)" srcset="doc/_static/images/logo-readme-dark.svg">
+         <source media="(prefers-color-scheme: light)" srcset="doc/_static/images/logo-readme-light.svg">
+         <img src="doc/_static/images/logo-readme-light.svg">
+       </picture>
      </p>
    </a>