DOC: Use URL as path to static content in README

Use commit raw content URL as path to static content in `README`.

Fixes:
```
Warning, treated as error:
/home/runner/work/eddymotion/eddymotion/README.rst::image file not readable:
docs/_static/eddymotion-flowchart.svg
make: *** [Makefile:65: html] Error 2
```

raised for example in:
https://github.com/nipreps/eddymotion/actions/runs/8391856943/job/22983140234#step:6:54

README.rst

@@ -44,7 +44,7 @@ the work of ``eddy`` and *SHORELine*, while generalizing these methods to multip
 (single-shell, multi-shell, and diffusion spectrum imaging) using diffusion models available with DIPY [#r5]_.
 
 
-.. image:: docs/_static/eddymotion-flowchart.svg
+.. image:: https://raw.githubusercontent.com/nipreps/eddymotion/507fc9bab86696d5330fd6a86c3870968243aea8/docs/_static/eddymotion-flowchart.svg
    :alt: The eddymotion flowchart