Fix contributor dependency installation instructions

[jan-glx]

Mirroring scverse/spatialdata#1174 this PR replaces the obsolete editable-install extras syntax in the pip contribution instructions with pip dependency groups.

Development dependencies are declared under [dependency-groups], not [project.optional-dependencies]. As a result, following the current instructions, pip install -e ".[dev,test,doc]" warns that those extras do not exist and does not install the contributor dependencies.
The updated instructions uses groups: pip install -e . --group dev --group test --group doc - which will actually install the declared dependencies.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.93%. Comparing base (8bcfe6c) to head (7f9d3b4).

Additional details and impacted files

@@ Coverage Diff @@## main #769 +/- ##
=======================================
Coverage 79.93% 79.93% =======================================
Files 18 18 Lines 4814 4814 Branches 1069 1069 =======================================
Hits 3848 3848 Misses 604 604 Partials 362 362 

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[timtreis]

Thanks @jan-glx !

[timtreis]

lgtm