[doc] Serve the DOI badge image from shields.io

Zenodo sends the badge SVG with cache-control: no-cache behind a
120 req/min per-IP rate limit. GitHub's camo proxy therefore re-fetches
it on every render from a shared IP pool, often getting a 429 that it
surfaces as a 502 broken image.

shields.io sends max-age=432000, so camo caches it instead. The link
target stays the concept DOI.

Assisted-by: Claude <noreply@anthropic.com>

Author: Wentzell

README.md

@@ -1,4 +1,4 @@
-[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.21691748.svg)](https://doi.org/10.5281/zenodo.21691748)
+[![DOI](https://img.shields.io/badge/DOI-10.5281%2Fzenodo.21691748-blue.svg)](https://doi.org/10.5281/zenodo.21691748)
 
 DFT_Tools - A TRIQS application for ab initio calculations