docs(readme): use adaptive dj.Diagram pipeline figure

Point the README at the restyled, theme-adaptive images/pipeline.svg (modernized
dj.Diagram notation; Activity as a Part of Segmentation with cell_mask/trace on
the part and make() inserting into it).

Author: dimitri-yatsenko

README.md

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