Fix contributor dependency installation instructions (docs only)

[jan-glx]

The current contribution install instructions:

pip install -e ".[dev,test,doc]"

warn:

WARNING: spatialdata 0.8.1.dev2+geb4fb3d23 does not provide the extra 'dev'
WARNING: spatialdata 0.8.1.dev2+geb4fb3d23 does not provide the extra 'test'
WARNING: spatialdata 0.8.1.dev2+geb4fb3d23 does not provide the extra 'doc'

and do not install the development dependencies. This PR updates the instructions to use pip dependency groups, following their introduction in cdf3435f.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.44%. Comparing base (eb4fb3d) to head (bf439f1).

Additional details and impacted files

@@ Coverage Diff @@## main #1174 +/- ##
=======================================
Coverage 92.44% 92.44% =======================================
Files 51 51 Lines 7820 7820 =======================================
Hits 7229 7229 Misses 591 591 

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[jan-glx]

@ajkswamy found the same same issue in https://github.com/scverse/spatialdata-plot/blob/main/docs/contributing.md

[Zethson]

@jan-glx would be willing to make a PR to spatialdata-plot, please? CC @timtreis