docs(gallery): add normalization and contrast tutorial

[timtreis]

Summary

Links the new Normalization and contrast tutorial into the docs gallery, mirroring the source notebook added in scverse/spatialdata-plot-notebooks#9.

• adds a Tutorials card to docs/gallery.md
• adds the thumbnail docs/_static/gallery/normalization_and_contrast.png (same image used in the notebooks repo)
• bumps the docs/notebooks submodule to the branch carrying the tutorial so the page resolves

The tutorial covers the norm= argument end-to-end: fixed contrast limits, clip under/over vs clamp, LogNorm/PowerNorm/SymLogNorm, percentile contrast via PercentileNormalize, per-channel norm lists for images, and norms on shapes/points/labels.

Merge order

1. Merge Add normalization and contrast tutorial spatialdata-plot-notebooks#9 first.
2. Re-point the docs/notebooks submodule here to notebooks main (currently it references the feature-branch commit so this PR builds).

PercentileNormalize ships with the next spatialdata-plot release (#725, on main).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.20%. Comparing base (0919069) to head (7eb6062).

Additional details and impacted files

@@           Coverage Diff           @@
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  Coverage   79.20%   79.20%           
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  Files          17       17           
  Lines        4563     4563           
  Branches     1026     1026           
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  Hits         3614     3614           
  Misses        599      599           
  Partials      350      350           

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