docs: refresh README pipeline illustration

[dimitri-yatsenko]

Summary

• Replaces the pre-2.0 pipeline illustration in the README with the same new asset that lands in datajoint-docs (datajoint-docs#164).
• Adds images/pipeline.svg as the editable source of truth — text in the repo, so the source can't go missing again.
• Re-renders images/pipeline.png from the SVG so the README's existing raw-URL image link keeps working without edits. (GitHub serves raw .svg as text/plain, which doesn't render inline; PNG stays the safer display format for README.)
• Removes images/pipeline.drawio — the stale source for the old PNG, now superseded by the SVG.

The new illustration uses calcium-imaging vocabulary matching the docs (Subject → Session → Scan → AverageFrame → Segmentation → Activity), follows DataJoint's notation (rectangles for Manual/Lookup, ovals for Imported/Computed), and shows 2.0 type syntax (int32, <blob@>).

Test plan

• View the rendered README on this PR — confirm the new pipeline image displays correctly.
• Verify shape convention: rectangles for Subject/Session/Scan/SegmentationParams; ovals for AverageFrame/Segmentation/Activity.
• Verify code snippet uses int32 and <blob@>, not longblob/double.
• Confirm no other docs or scripts reference pipeline.drawio (verified via grep at commit time).

[MilagrosMarin]

@dimitri-yatsenko, thanks for this PR. I have a couple of comments: (1) It seems the new image is much smaller than the previous one. Can you confirm it is visible and clear, with a font size of approximately 9-11 points? (2) In addition, I see that the comments on the right are set against a transparent background, while the make function text has a white background. This can be an issue if the background turns black, since the text is black too. Could these comments be set against a white background as well?

[dimitri-yatsenko]

Thanks for the careful review, @MilagrosMarin — both points were on the money.

Pushed db98bfa to address them:

1. Added an opaque white <rect> covering the full SVG viewBox so the entire illustration (annotations included) renders against white regardless of page theme. The transparent gutters around the code panel made the right-side annotations invisible in GitHub's dark-mode README rendering — confirmed and fixed.
2. Bumped the annotation font size from 17 to 19. At a typical README column width of ~800 px, that puts effective rendered size around 10 pt — within the 9–11 pt range you flagged. The diagram labels (22) and code (18) were already in range.

Re-rendered images/pipeline.png from the updated SVG, so the README image link reflects the fix without further edits. The same change is going up shortly in datajoint-docs as a follow-up PR.