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

Zenodo serves its badge SVG as a dynamic UI response
(cache-control: no-cache, session cookie) behind a 120 req/min per-IP
rate limit. GitHub's camo proxy therefore cannot cache it and re-fetches
on every render from a shared egress IP pool whose quota is chronically
exhausted, so a large fraction of renders got a 429 from Zenodo, which
camo surfaces as a 502 and the browser renders as a broken image.

shields.io sends cache-control: max-age=86400, so camo caches the image
and never re-hits the origin. logo=zenodo keeps the Zenodo mark, inlined
as a data: URI so it costs no additional request. The link target stays
the concept DOI, which never needs bumping per release.

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

Author: Wentzell

README.md

@@ -4,7 +4,7 @@
 ![CI stable](https://github.com/triqs/solid_dmft/actions/workflows/build_release.yml/badge.svg)
 [![PyPI version](https://badge.fury.io/py/solid_dmft.svg)](https://badge.fury.io/py/solid_dmft)
 [![status](https://joss.theoj.org/papers/48eb529b08c6bb464b235ba919d78922/status.svg)](https://joss.theoj.org/papers/48eb529b08c6bb464b235ba919d78922)
-[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.7105085.svg)](https://doi.org/10.5281/zenodo.7105085)
+[![DOI](https://img.shields.io/badge/DOI-10.5281%2Fzenodo.7105085-blue.svg)](https://doi.org/10.5281/zenodo.7105085)
 
 
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