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