README: serve the DOI badge from shields.io

The zenodo.org badge endpoint is slow enough that GitHub's Camo proxy
intermittently times out and caches the failure, so the badge renders as a
broken image. A static shields.io badge shows the same concept DOI with the
same doi.org link, without the Zenodo dependency.

Author: Sébastien Loisel

README.md

@@ -4,7 +4,7 @@
 [![Dev](https://img.shields.io/badge/docs-dev-blue.svg)](https://sloisel.github.io/MultiGridBarrier.jl/dev/)
 [![Build Status](https://github.com/sloisel/MultiGridBarrier.jl/actions/workflows/CI.yml/badge.svg?branch=main)](https://github.com/sloisel/MultiGridBarrier.jl/actions/workflows/CI.yml?query=branch%3Amain)
 [![Coverage](https://codecov.io/gh/sloisel/MultiGridBarrier.jl/branch/main/graph/badge.svg)](https://codecov.io/gh/sloisel/MultiGridBarrier.jl)
-[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.20610950.svg)](https://doi.org/10.5281/zenodo.20610950)
+[![DOI](https://img.shields.io/badge/DOI-10.5281%2Fzenodo.20610950-blue.svg)](https://doi.org/10.5281/zenodo.20610950)
 
 <p align="center">
   <img src="docs/src/assets/hero.gif" alt="A nonsmooth p-Laplace solution computed with MultiGridBarrier.jl" width="460">