docs: use relative path for README pipeline image

The absolute raw.githubusercontent.com URL is served via GitHub's camo
image proxy, which caches aggressively — so even after #1448 refreshed
images/pipeline.png, the rendered README kept showing the old image.

Switching to a relative path lets GitHub serve the image directly from
the current commit, so the new illustration appears immediately and
future updates to the file render without cache hiccups.

Author: MilagrosMarin

README.md

@@ -69,7 +69,7 @@ conda install -c conda-forge datajoint
 
 ## Example Pipeline
 
-![pipeline](https://raw.githubusercontent.com/datajoint/datajoint-python/master/images/pipeline.png)
+![pipeline](images/pipeline.png)
 
 **Cite DataJoint:** [Yatsenko et al., 2026](https://arxiv.org/abs/2602.16585) — RRID: [SCR_014543](https://scicrunch.org/resolver/SCR_014543)